diff --git a/icu4c/source/common/brkeng.h b/icu4c/source/common/brkeng.h
index f6b64c83e25..6843f1cc953 100644
--- a/icu4c/source/common/brkeng.h
+++ b/icu4c/source/common/brkeng.h
@@ -176,7 +176,7 @@ class UnhandledEngine : public LanguageBreakEngine {
   * @return true if this engine handles the particular character and break
   * type.
   */
-  virtual UBool handles(UChar32 c) const;
+  virtual UBool handles(UChar32 c) const override;
 
  /**
   * <p>Find any breaks within a run in the supplied text.</p>
@@ -194,7 +194,7 @@ class UnhandledEngine : public LanguageBreakEngine {
                               int32_t startPos,
                               int32_t endPos,
                               UVector32 &foundBreaks,
-                              UErrorCode &status) const;
+                              UErrorCode &status) const override;
 
  /**
   * <p>Tell the engine to handle a particular character and break type.</p>
@@ -247,7 +247,7 @@ class ICULanguageBreakFactory : public LanguageBreakFactory {
   * sought.
   * @return A LanguageBreakEngine with the desired characteristics, or 0.
   */
-  virtual const LanguageBreakEngine *getEngineFor(UChar32 c);
+  virtual const LanguageBreakEngine *getEngineFor(UChar32 c) override;
 
 protected:
  /**
diff --git a/icu4c/source/common/brkiter.cpp b/icu4c/source/common/brkiter.cpp
index b9b6ca65cd4..8b228acf2c3 100644
--- a/icu4c/source/common/brkiter.cpp
+++ b/icu4c/source/common/brkiter.cpp
@@ -234,7 +234,7 @@ class ICUBreakIteratorFactory : public ICUResourceBundleFactory {
 public:
     virtual ~ICUBreakIteratorFactory();
 protected:
-    virtual UObject* handleCreate(const Locale& loc, int32_t kind, const ICUService* /*service*/, UErrorCode& status) const {
+    virtual UObject* handleCreate(const Locale& loc, int32_t kind, const ICUService* /*service*/, UErrorCode& status) const override {
         return BreakIterator::makeInstance(loc, kind, status);
     }
 };
@@ -254,11 +254,11 @@ public:
 
     virtual ~ICUBreakIteratorService();
 
-    virtual UObject* cloneInstance(UObject* instance) const {
+    virtual UObject* cloneInstance(UObject* instance) const override {
         return ((BreakIterator*)instance)->clone();
     }
 
-    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* /*actualID*/, UErrorCode& status) const {
+    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* /*actualID*/, UErrorCode& status) const override {
         LocaleKey& lkey = (LocaleKey&)key;
         int32_t kind = lkey.kind();
         Locale loc;
@@ -266,7 +266,7 @@ public:
         return BreakIterator::makeInstance(loc, kind, status);
     }
 
-    virtual UBool isDefault() const {
+    virtual UBool isDefault() const override {
         return countFactories() == 1;
     }
 };
diff --git a/icu4c/source/common/dictbe.h b/icu4c/source/common/dictbe.h
index 4adaaa4f09d..4e70ed38171 100644
--- a/icu4c/source/common/dictbe.h
+++ b/icu4c/source/common/dictbe.h
@@ -62,7 +62,7 @@ class DictionaryBreakEngine : public LanguageBreakEngine {
    * @return true if this engine handles the particular character and break
    * type.
    */
-  virtual UBool handles(UChar32 c) const;
+  virtual UBool handles(UChar32 c) const override;
 
   /**
    * <p>Find any breaks within a run in the supplied text.</p>
@@ -80,7 +80,7 @@ class DictionaryBreakEngine : public LanguageBreakEngine {
                               int32_t startPos,
                               int32_t endPos,
                               UVector32 &foundBreaks,
-                              UErrorCode& status ) const;
+                              UErrorCode& status ) const override;
 
  protected:
 
@@ -164,7 +164,7 @@ class ThaiBreakEngine : public DictionaryBreakEngine {
                                            int32_t rangeStart,
                                            int32_t rangeEnd,
                                            UVector32 &foundBreaks,
-                                           UErrorCode& status) const;
+                                           UErrorCode& status) const override;
 
 };
 
@@ -222,7 +222,7 @@ class LaoBreakEngine : public DictionaryBreakEngine {
                                            int32_t rangeStart,
                                            int32_t rangeEnd,
                                            UVector32 &foundBreaks,
-                                           UErrorCode& status) const;
+                                           UErrorCode& status) const override;
 
 };
 
@@ -280,7 +280,7 @@ class BurmeseBreakEngine : public DictionaryBreakEngine {
                                            int32_t rangeStart,
                                            int32_t rangeEnd,
                                            UVector32 &foundBreaks,
-                                           UErrorCode& status) const;
+                                           UErrorCode& status) const override;
 
 };
 
@@ -338,7 +338,7 @@ class KhmerBreakEngine : public DictionaryBreakEngine {
                                            int32_t rangeStart,
                                            int32_t rangeEnd,
                                            UVector32 &foundBreaks,
-                                           UErrorCode& status) const;
+                                           UErrorCode& status) const override;
 
 };
 
@@ -404,7 +404,7 @@ class CjkBreakEngine : public DictionaryBreakEngine {
           int32_t rangeStart,
           int32_t rangeEnd,
           UVector32 &foundBreaks,
-          UErrorCode& status) const;
+          UErrorCode& status) const override;
 
 };
 
diff --git a/icu4c/source/common/dictionarydata.h b/icu4c/source/common/dictionarydata.h
index 0d303d9a8dc..33e4fdc1065 100644
--- a/icu4c/source/common/dictionarydata.h
+++ b/icu4c/source/common/dictionarydata.h
@@ -107,8 +107,8 @@ public:
     virtual ~UCharsDictionaryMatcher();
     virtual int32_t matches(UText *text, int32_t maxLength, int32_t limit,
                             int32_t *lengths, int32_t *cpLengths, int32_t *values,
-                            int32_t *prefix) const;
-    virtual int32_t getType() const;
+                            int32_t *prefix) const override;
+    virtual int32_t getType() const override;
 private:
     const UChar *characters;
     UDataMemory *file;
@@ -125,8 +125,8 @@ public:
     virtual ~BytesDictionaryMatcher();
     virtual int32_t matches(UText *text, int32_t maxLength, int32_t limit,
                             int32_t *lengths, int32_t *cpLengths, int32_t *values,
-                            int32_t *prefix) const;
-    virtual int32_t getType() const;
+                            int32_t *prefix) const override;
+    virtual int32_t getType() const override;
 private:
     UChar32 transform(UChar32 c) const;
 
diff --git a/icu4c/source/common/filteredbrk.cpp b/icu4c/source/common/filteredbrk.cpp
index 9bced688702..e4817367a5a 100644
--- a/icu4c/source/common/filteredbrk.cpp
+++ b/icu4c/source/common/filteredbrk.cpp
@@ -185,37 +185,37 @@ public:
   /* -- cloning and other subclass stuff -- */
   virtual BreakIterator *  createBufferClone(void * /*stackBuffer*/,
                                              int32_t &/*BufferSize*/,
-                                             UErrorCode &status) {
+                                             UErrorCode &status) override {
     // for now - always deep clone
     status = U_SAFECLONE_ALLOCATED_WARNING;
     return clone();
   }
-  virtual SimpleFilteredSentenceBreakIterator* clone() const { return new SimpleFilteredSentenceBreakIterator(*this); }
-  virtual UClassID getDynamicClassID(void) const { return NULL; }
-  virtual bool operator==(const BreakIterator& o) const { if(this==&o) return true; return false; }
+  virtual SimpleFilteredSentenceBreakIterator* clone() const override { return new SimpleFilteredSentenceBreakIterator(*this); }
+  virtual UClassID getDynamicClassID(void) const override { return NULL; }
+  virtual bool operator==(const BreakIterator& o) const override { if(this==&o) return true; return false; }
 
   /* -- text modifying -- */
-  virtual void setText(UText *text, UErrorCode &status) { fDelegate->setText(text,status); }
-  virtual BreakIterator &refreshInputText(UText *input, UErrorCode &status) { fDelegate->refreshInputText(input,status); return *this; }
-  virtual void adoptText(CharacterIterator* it) { fDelegate->adoptText(it); }
-  virtual void setText(const UnicodeString &text) { fDelegate->setText(text); }
+  virtual void setText(UText *text, UErrorCode &status) override { fDelegate->setText(text,status); }
+  virtual BreakIterator &refreshInputText(UText *input, UErrorCode &status) override { fDelegate->refreshInputText(input,status); return *this; }
+  virtual void adoptText(CharacterIterator* it) override { fDelegate->adoptText(it); }
+  virtual void setText(const UnicodeString &text) override { fDelegate->setText(text); }
 
   /* -- other functions that are just delegated -- */
-  virtual UText *getUText(UText *fillIn, UErrorCode &status) const { return fDelegate->getUText(fillIn,status); }
-  virtual CharacterIterator& getText(void) const { return fDelegate->getText(); }
+  virtual UText *getUText(UText *fillIn, UErrorCode &status) const override { return fDelegate->getUText(fillIn,status); }
+  virtual CharacterIterator& getText(void) const override { return fDelegate->getText(); }
 
   /* -- ITERATION -- */
-  virtual int32_t first(void);
-  virtual int32_t preceding(int32_t offset);
-  virtual int32_t previous(void);
-  virtual UBool isBoundary(int32_t offset);
-  virtual int32_t current(void) const { return fDelegate->current(); } // we keep the delegate current, so this should be correct.
+  virtual int32_t first(void) override;
+  virtual int32_t preceding(int32_t offset) override;
+  virtual int32_t previous(void) override;
+  virtual UBool isBoundary(int32_t offset) override;
+  virtual int32_t current(void) const override { return fDelegate->current(); } // we keep the delegate current, so this should be correct.
 
-  virtual int32_t next(void);
+  virtual int32_t next(void) override;
 
-  virtual int32_t next(int32_t n);
-  virtual int32_t following(int32_t offset);
-  virtual int32_t last(void);
+  virtual int32_t next(int32_t n) override;
+  virtual int32_t following(int32_t offset) override;
+  virtual int32_t last(void) override;
 
 private:
     /**
@@ -487,9 +487,9 @@ public:
   virtual ~SimpleFilteredBreakIteratorBuilder();
   SimpleFilteredBreakIteratorBuilder(const Locale &fromLocale, UErrorCode &status);
   SimpleFilteredBreakIteratorBuilder(UErrorCode &status);
-  virtual UBool suppressBreakAfter(const UnicodeString& exception, UErrorCode& status);
-  virtual UBool unsuppressBreakAfter(const UnicodeString& exception, UErrorCode& status);
-  virtual BreakIterator *build(BreakIterator* adoptBreakIterator, UErrorCode& status);
+  virtual UBool suppressBreakAfter(const UnicodeString& exception, UErrorCode& status) override;
+  virtual UBool unsuppressBreakAfter(const UnicodeString& exception, UErrorCode& status) override;
+  virtual BreakIterator *build(BreakIterator* adoptBreakIterator, UErrorCode& status) override;
 private:
   UStringSet fSet;
 };
diff --git a/icu4c/source/common/locdspnm.cpp b/icu4c/source/common/locdspnm.cpp
index 43334f51964..c4c04702e64 100644
--- a/icu4c/source/common/locdspnm.cpp
+++ b/icu4c/source/common/locdspnm.cpp
@@ -313,29 +313,29 @@ public:
     LocaleDisplayNamesImpl(const Locale& locale, UDisplayContext *contexts, int32_t length);
     virtual ~LocaleDisplayNamesImpl();
 
-    virtual const Locale& getLocale() const;
-    virtual UDialectHandling getDialectHandling() const;
-    virtual UDisplayContext getContext(UDisplayContextType type) const;
+    virtual const Locale& getLocale() const override;
+    virtual UDialectHandling getDialectHandling() const override;
+    virtual UDisplayContext getContext(UDisplayContextType type) const override;
 
     virtual UnicodeString& localeDisplayName(const Locale& locale,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& localeDisplayName(const char* localeId,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& languageDisplayName(const char* lang,
-                                               UnicodeString& result) const;
+                                               UnicodeString& result) const override;
     virtual UnicodeString& scriptDisplayName(const char* script,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& scriptDisplayName(UScriptCode scriptCode,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& regionDisplayName(const char* region,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& variantDisplayName(const char* variant,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& keyDisplayName(const char* key,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
     virtual UnicodeString& keyValueDisplayName(const char* key,
                                                 const char* value,
-                                                UnicodeString& result) const;
+                                                UnicodeString& result) const override;
 private:
     UnicodeString& localeIdName(const char* localeId,
                                 UnicodeString& result, bool substitute) const;
@@ -407,7 +407,7 @@ struct LocaleDisplayNamesImpl::CapitalizationContextSink : public ResourceSink {
     virtual ~CapitalizationContextSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         ResourceTable contexts = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
         for (int i = 0; contexts.getKeyAndValue(i, key, value); ++i) {
diff --git a/icu4c/source/common/locid.cpp b/icu4c/source/common/locid.cpp
index cff1d07e1c9..e8859c7048b 100644
--- a/icu4c/source/common/locid.cpp
+++ b/icu4c/source/common/locid.cpp
@@ -2418,7 +2418,7 @@ private:
 
 public:
     static UClassID U_EXPORT2 getStaticClassID(void) { return (UClassID)&fgClassID; }
-    virtual UClassID getDynamicClassID(void) const { return getStaticClassID(); }
+    virtual UClassID getDynamicClassID(void) const override { return getStaticClassID(); }
 public:
     KeywordEnumeration(const char *keys, int32_t keywordLen, int32_t currentIndex, UErrorCode &status)
         : keywords((char *)&fgClassID), current((char *)&fgClassID), length(0) {
@@ -2442,13 +2442,13 @@ public:
 
     virtual ~KeywordEnumeration();
 
-    virtual StringEnumeration * clone() const
+    virtual StringEnumeration * clone() const override
     {
         UErrorCode status = U_ZERO_ERROR;
         return new KeywordEnumeration(keywords, length, (int32_t)(current - keywords), status);
     }
 
-    virtual int32_t count(UErrorCode &/*status*/) const {
+    virtual int32_t count(UErrorCode &/*status*/) const override {
         char *kw = keywords;
         int32_t result = 0;
         while(*kw) {
@@ -2458,7 +2458,7 @@ public:
         return result;
     }
 
-    virtual const char* next(int32_t* resultLength, UErrorCode& status) {
+    virtual const char* next(int32_t* resultLength, UErrorCode& status) override {
         const char* result;
         int32_t len;
         if(U_SUCCESS(status) && *current != 0) {
@@ -2477,13 +2477,13 @@ public:
         return result;
     }
 
-    virtual const UnicodeString* snext(UErrorCode& status) {
+    virtual const UnicodeString* snext(UErrorCode& status) override {
         int32_t resultLength = 0;
         const char *s = next(&resultLength, status);
         return setChars(s, resultLength, status);
     }
 
-    virtual void reset(UErrorCode& /*status*/) {
+    virtual void reset(UErrorCode& /*status*/) override {
         current = keywords;
     }
 };
@@ -2501,7 +2501,7 @@ public:
     using KeywordEnumeration::KeywordEnumeration;
     virtual ~UnicodeKeywordEnumeration();
 
-    virtual const char* next(int32_t* resultLength, UErrorCode& status) {
+    virtual const char* next(int32_t* resultLength, UErrorCode& status) override {
         const char* legacy_key = KeywordEnumeration::next(nullptr, status);
         while (U_SUCCESS(status) && legacy_key != nullptr) {
             const char* key = uloc_toUnicodeLocaleKey(legacy_key);
diff --git a/icu4c/source/common/lstmbe.cpp b/icu4c/source/common/lstmbe.cpp
index df742c812b3..2987da003d1 100644
--- a/icu4c/source/common/lstmbe.cpp
+++ b/icu4c/source/common/lstmbe.cpp
@@ -93,8 +93,8 @@ public:
     }
 
     // ReadArray1D methods.
-    virtual int32_t d1() const { return d1_; }
-    virtual float get(int32_t i) const {
+    virtual int32_t d1() const override { return d1_; }
+    virtual float get(int32_t i) const override {
         U_ASSERT(i < d1_);
         return data_[i];
     }
@@ -131,9 +131,9 @@ public:
     }
 
     // ReadArray2D methods.
-    inline int32_t d1() const { return d1_; }
-    inline int32_t d2() const { return d2_; }
-    float get(int32_t i, int32_t j) const {
+    inline int32_t d1() const override { return d1_; }
+    inline int32_t d2() const override { return d2_; }
+    float get(int32_t i, int32_t j) const override {
         U_ASSERT(i < d1_);
         U_ASSERT(j < d2_);
         return data_[i * d2_ + j];
@@ -182,8 +182,8 @@ public:
         : memory_(nullptr), data_(data), d1_(d1) {}
 
     // ReadArray1D methods.
-    virtual int32_t d1() const { return d1_; }
-    virtual float get(int32_t i) const {
+    virtual int32_t d1() const override { return d1_; }
+    virtual float get(int32_t i) const override {
         U_ASSERT(i < d1_);
         return data_[i];
     }
@@ -313,9 +313,9 @@ public:
     virtual ~Array2D();
 
     // ReadArray2D methods.
-    virtual int32_t d1() const { return d1_; }
-    virtual int32_t d2() const { return d2_; }
-    virtual float get(int32_t i, int32_t j) const {
+    virtual int32_t d1() const override { return d1_; }
+    virtual int32_t d2() const override { return d2_; }
+    virtual float get(int32_t i, int32_t j) const override {
         U_ASSERT(i < d1_);
         U_ASSERT(j < d2_);
         return data_[i * d2_ + j];
@@ -508,7 +508,7 @@ public:
     virtual ~CodePointsVectorizer();
     virtual void vectorize(UText *text, int32_t startPos, int32_t endPos,
                            UVector32 &offsets, UVector32 &indices,
-                           UErrorCode &status) const;
+                           UErrorCode &status) const override;
 };
 
 CodePointsVectorizer::~CodePointsVectorizer()
@@ -547,7 +547,7 @@ public:
     virtual ~GraphemeClusterVectorizer();
     virtual void vectorize(UText *text, int32_t startPos, int32_t endPos,
                            UVector32 &offsets, UVector32 &indices,
-                           UErrorCode &status) const;
+                           UErrorCode &status) const override;
 };
 
 GraphemeClusterVectorizer::~GraphemeClusterVectorizer()
diff --git a/icu4c/source/common/lstmbe.h b/icu4c/source/common/lstmbe.h
index 84015e3a1fc..c3f7ecf8154 100644
--- a/icu4c/source/common/lstmbe.h
+++ b/icu4c/source/common/lstmbe.h
@@ -62,7 +62,7 @@ protected:
                                              int32_t rangeStart,
                                              int32_t rangeEnd,
                                              UVector32 &foundBreaks,
-                                             UErrorCode& status) const;
+                                             UErrorCode& status) const override;
 private:
     const LSTMData* fData;
     const Vectorizer* fVectorizer;
diff --git a/icu4c/source/common/rbbirb.h b/icu4c/source/common/rbbirb.h
index 037c1dc2ce8..11dbf343ece 100644
--- a/icu4c/source/common/rbbirb.h
+++ b/icu4c/source/common/rbbirb.h
@@ -73,10 +73,10 @@ private:
 
 public:
     //  API inherited from class SymbolTable
-    virtual const UnicodeString*  lookup(const UnicodeString& s) const;
-    virtual const UnicodeFunctor* lookupMatcher(UChar32 ch) const;
+    virtual const UnicodeString*  lookup(const UnicodeString& s) const override;
+    virtual const UnicodeFunctor* lookupMatcher(UChar32 ch) const override;
     virtual UnicodeString parseReference(const UnicodeString& text,
-                                         ParsePosition& pos, int32_t limit) const;
+                                         ParsePosition& pos, int32_t limit) const override;
 
     //  Additional Functions
     RBBISymbolTable(RBBIRuleScanner *, const UnicodeString &fRules, UErrorCode &status);
diff --git a/icu4c/source/common/serv.h b/icu4c/source/common/serv.h
index 4a6a0fffb82..3bd3d9a9b9e 100644
--- a/icu4c/source/common/serv.h
+++ b/icu4c/source/common/serv.h
@@ -191,7 +191,7 @@ public:
   /**
    * UObject RTTI boilerplate.
    */
-  virtual UClassID getDynamicClassID() const;
+  virtual UClassID getDynamicClassID() const override;
 
 #ifdef SERVICE_DEBUG
  public:
@@ -315,7 +315,7 @@ class U_COMMON_API SimpleFactory : public ICUServiceFactory {
    * @param status the error code status.
    * @return the service object, or NULL if the factory does not support the key.
    */
-  virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const;
+  virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const override;
 
   /**
    * <p>This implementation adds a mapping from ID -> this to result if visible is true, 
@@ -324,7 +324,7 @@ class U_COMMON_API SimpleFactory : public ICUServiceFactory {
    * @param result the mapping table to update.
    * @param status the error code status.
    */
-  virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const;
+  virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const override;
 
   /**
    * <p>This implementation returns the factory ID if it equals id and visible is true,
@@ -336,7 +336,7 @@ class U_COMMON_API SimpleFactory : public ICUServiceFactory {
    * @param result output parameter to hold the display name.
    * @return result.
    */
-  virtual UnicodeString& getDisplayName(const UnicodeString& id, const Locale& locale, UnicodeString& result) const;
+  virtual UnicodeString& getDisplayName(const UnicodeString& id, const Locale& locale, UnicodeString& result) const override;
 
 public:
  /**
@@ -347,7 +347,7 @@ public:
  /**
   * UObject RTTI boilerplate.
   */
-  virtual UClassID getDynamicClassID() const;
+  virtual UClassID getDynamicClassID() const override;
 
 #ifdef SERVICE_DEBUG
  public:
@@ -390,7 +390,7 @@ public:
     /**
      * UObject RTTI boilerplate.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
     
 };
 
@@ -930,7 +930,7 @@ class U_COMMON_API ICUService : public ICUNotifier {
      * @param l the listener to test.
      * @return true if the service accepts the listener.
      */
-    virtual UBool acceptsListener(const EventListener& l) const;
+    virtual UBool acceptsListener(const EventListener& l) const override;
 
     /**
      * <p>Notify the listener of a service change.</p>
@@ -941,7 +941,7 @@ class U_COMMON_API ICUService : public ICUNotifier {
      *
      * @param l the listener to notify.
      */
-    virtual void notifyListener(EventListener& l) const;
+    virtual void notifyListener(EventListener& l) const override;
 
     /************************************************************************
      * Utilities for subclasses.
diff --git a/icu4c/source/common/servloc.h b/icu4c/source/common/servloc.h
index ccf6433379d..29c50a27d1a 100644
--- a/icu4c/source/common/servloc.h
+++ b/icu4c/source/common/servloc.h
@@ -106,7 +106,7 @@ class U_COMMON_API LocaleKey : public ICUServiceKey {
     /**
      * Append the prefix associated with the kind, or nothing if the kind is KIND_ANY.
      */
-    virtual UnicodeString& prefix(UnicodeString& result) const;
+    virtual UnicodeString& prefix(UnicodeString& result) const override;
 
     /**
      * Return the kind code associated with this key.
@@ -116,17 +116,17 @@ class U_COMMON_API LocaleKey : public ICUServiceKey {
     /**
      * Return the canonicalID.
      */
-    virtual UnicodeString& canonicalID(UnicodeString& result) const;
+    virtual UnicodeString& canonicalID(UnicodeString& result) const override;
 
     /**
      * Return the currentID.
      */
-    virtual UnicodeString& currentID(UnicodeString& result) const;
+    virtual UnicodeString& currentID(UnicodeString& result) const override;
 
     /**
      * Return the (canonical) current descriptor, or null if no current id.
      */
-    virtual UnicodeString& currentDescriptor(UnicodeString& result) const;
+    virtual UnicodeString& currentDescriptor(UnicodeString& result) const override;
 
     /**
      * Convenience method to return the locale corresponding to the (canonical) original ID.
@@ -147,13 +147,13 @@ class U_COMMON_API LocaleKey : public ICUServiceKey {
      * unless the primary id was the empty string, in which case
      * there is no fallback.  
      */
-    virtual UBool fallback();
+    virtual UBool fallback() override;
 
     /**
      * Return true if a key created from id matches, or would eventually
      * fallback to match, the canonical ID of this key.  
      */
-    virtual UBool isFallbackOf(const UnicodeString& id) const;
+    virtual UBool isFallbackOf(const UnicodeString& id) const override;
     
  public:
     /**
@@ -161,7 +161,7 @@ class U_COMMON_API LocaleKey : public ICUServiceKey {
      */
     static UClassID U_EXPORT2 getStaticClassID();
 
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * Destructor.
@@ -238,7 +238,7 @@ protected:
      * kind off to handleCreate (which subclasses must implement).
      */
 public:
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const;
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const override;
 
 protected:
     virtual UBool handlesKey(const ICUServiceKey& key, UErrorCode& status) const;
@@ -248,12 +248,12 @@ public:
      * Override of superclass method.  This adjusts the result based
      * on the coverage rule for this factory.
      */
-    virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const;
+    virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const override;
 
     /**
      * Return a localized name for the locale represented by id.
      */
-    virtual UnicodeString& getDisplayName(const UnicodeString& id, const Locale& locale, UnicodeString& result) const;
+    virtual UnicodeString& getDisplayName(const UnicodeString& id, const Locale& locale, UnicodeString& result) const override;
 
 protected:
     /**
@@ -281,7 +281,7 @@ public:
      */
     static UClassID U_EXPORT2 getStaticClassID();
 
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 #ifdef SERVICE_DEBUG
  public:
@@ -324,13 +324,13 @@ class U_COMMON_API SimpleLocaleKeyFactory : public LocaleKeyFactory {
     /**
      * Override of superclass method.  Returns the service object if kind/locale match.  Service is not used.
      */
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const;
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const override;
 
     /**
      * Override of superclass method.  This adjusts the result based
      * on the coverage rule for this factory.
      */
-    virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const;
+    virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const override;
 
  protected:
     /**
@@ -345,7 +345,7 @@ public:
      */
     static UClassID U_EXPORT2 getStaticClassID();
 
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 #ifdef SERVICE_DEBUG
  public:
@@ -394,20 +394,20 @@ protected:
     /**
      * Return the supported IDs.  This is the set of all locale names in ICULocaleData.
      */
-    virtual const Hashtable* getSupportedIDs(UErrorCode& status) const;
+    virtual const Hashtable* getSupportedIDs(UErrorCode& status) const override;
 
     /**
      * Create the service.  The default implementation returns the resource bundle
      * for the locale, ignoring kind, and service.
      */
-    virtual UObject* handleCreate(const Locale& loc, int32_t kind, const ICUService* service, UErrorCode& status) const;
+    virtual UObject* handleCreate(const Locale& loc, int32_t kind, const ICUService* service, UErrorCode& status) const override;
 
 public:
     /**
      * UObject boilerplate.
      */
     static UClassID U_EXPORT2 getStaticClassID();
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 
 #ifdef SERVICE_DEBUG
@@ -512,7 +512,7 @@ class U_COMMON_API ICULocaleService : public ICUService
    * We really need a flag that is understood by all compilers that will suppress the warning about
    * hidden overrides.
    */
-  virtual URegistryKey registerInstance(UObject* objToAdopt, const UnicodeString& locale, UBool visible, UErrorCode& status);
+  virtual URegistryKey registerInstance(UObject* objToAdopt, const UnicodeString& locale, UBool visible, UErrorCode& status) override;
 
   /**
    * Convenience method for callers using locales.  This returns the standard
@@ -531,7 +531,7 @@ class U_COMMON_API ICULocaleService : public ICUService
   /**
    * Override superclass createKey method.
    */
-  virtual ICUServiceKey* createKey(const UnicodeString* id, UErrorCode& status) const;
+  virtual ICUServiceKey* createKey(const UnicodeString* id, UErrorCode& status) const override;
 
   /**
    * Additional createKey that takes a kind.
diff --git a/icu4c/source/common/servls.cpp b/icu4c/source/common/servls.cpp
index b6e27507a6f..7108afd4a52 100644
--- a/icu4c/source/common/servls.cpp
+++ b/icu4c/source/common/servls.cpp
@@ -201,7 +201,7 @@ public:
 
     virtual ~ServiceEnumeration();
 
-    virtual StringEnumeration *clone() const {
+    virtual StringEnumeration *clone() const override {
         UErrorCode status = U_ZERO_ERROR;
         ServiceEnumeration *cl = new ServiceEnumeration(*this, status);
         if(U_FAILURE(status)) {
@@ -221,18 +221,18 @@ public:
         return FALSE;
     }
 
-    virtual int32_t count(UErrorCode& status) const {
+    virtual int32_t count(UErrorCode& status) const override {
         return upToDate(status) ? _ids.size() : 0;
     }
 
-    virtual const UnicodeString* snext(UErrorCode& status) {
+    virtual const UnicodeString* snext(UErrorCode& status) override {
         if (upToDate(status) && (_pos < _ids.size())) {
             return (const UnicodeString*)_ids[_pos++];
         }
         return NULL;
     }
 
-    virtual void reset(UErrorCode& status) {
+    virtual void reset(UErrorCode& status) override {
         if (status == U_ENUM_OUT_OF_SYNC_ERROR) {
             status = U_ZERO_ERROR;
         }
@@ -245,7 +245,7 @@ public:
 
 public:
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 ServiceEnumeration::~ServiceEnumeration() {}
diff --git a/icu4c/source/common/servnotf.h b/icu4c/source/common/servnotf.h
index 73ce38c772e..e3b2cac32e4 100644
--- a/icu4c/source/common/servnotf.h
+++ b/icu4c/source/common/servnotf.h
@@ -40,7 +40,7 @@ public:
 public:
     static UClassID U_EXPORT2 getStaticClassID();
 
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 public:
 #ifdef SERVICE_DEBUG
diff --git a/icu4c/source/common/unicode/appendable.h b/icu4c/source/common/unicode/appendable.h
index fc99254de14..f77df88e39c 100644
--- a/icu4c/source/common/unicode/appendable.h
+++ b/icu4c/source/common/unicode/appendable.h
@@ -174,7 +174,7 @@ public:
      * @return true if the operation succeeded
      * @stable ICU 4.8
      */
-    virtual UBool appendCodeUnit(char16_t c);
+    virtual UBool appendCodeUnit(char16_t c) override;
 
     /**
      * Appends a code point to the string.
@@ -182,7 +182,7 @@ public:
      * @return true if the operation succeeded
      * @stable ICU 4.8
      */
-    virtual UBool appendCodePoint(UChar32 c);
+    virtual UBool appendCodePoint(UChar32 c) override;
 
     /**
      * Appends a string to the UnicodeString.
@@ -191,7 +191,7 @@ public:
      * @return true if the operation succeeded
      * @stable ICU 4.8
      */
-    virtual UBool appendString(const char16_t *s, int32_t length);
+    virtual UBool appendString(const char16_t *s, int32_t length) override;
 
     /**
      * Tells the UnicodeString that the caller is going to append roughly
@@ -200,7 +200,7 @@ public:
      * @return true if the operation succeeded
      * @stable ICU 4.8
      */
-    virtual UBool reserveAppendCapacity(int32_t appendCapacity);
+    virtual UBool reserveAppendCapacity(int32_t appendCapacity) override;
 
     /**
      * Returns a writable buffer for appending and writes the buffer's capacity to
@@ -226,7 +226,7 @@ public:
     virtual char16_t *getAppendBuffer(int32_t minCapacity,
                                    int32_t desiredCapacityHint,
                                    char16_t *scratch, int32_t scratchCapacity,
-                                   int32_t *resultCapacity);
+                                   int32_t *resultCapacity) override;
 
 private:
     UnicodeString &str;
diff --git a/icu4c/source/common/unicode/bytestream.h b/icu4c/source/common/unicode/bytestream.h
index 9735ee0bf86..997746e4280 100644
--- a/icu4c/source/common/unicode/bytestream.h
+++ b/icu4c/source/common/unicode/bytestream.h
@@ -206,7 +206,7 @@ public:
    * @param n the number of bytes; must be non-negative
    * @stable ICU 4.2
    */
-  virtual void Append(const char* bytes, int32_t n);
+  virtual void Append(const char* bytes, int32_t n) override;
   /**
    * Returns a writable buffer for appending and writes the buffer's capacity to
    * *result_capacity. For details see the base class documentation.
@@ -224,7 +224,7 @@ public:
   virtual char* GetAppendBuffer(int32_t min_capacity,
                                 int32_t desired_capacity_hint,
                                 char* scratch, int32_t scratch_capacity,
-                                int32_t* result_capacity);
+                                int32_t* result_capacity) override;
   /**
    * Returns the number of bytes actually written to the sink.
    * @return number of bytes written to the buffer
@@ -291,7 +291,7 @@ class StringByteSink : public ByteSink {
    * @param n the number of bytes; must be non-negative
    * @stable ICU 4.2
    */
-  virtual void Append(const char* data, int32_t n) { dest_->append(data, n); }
+  virtual void Append(const char* data, int32_t n) override { dest_->append(data, n); }
  private:
   StringClass* dest_;
 
diff --git a/icu4c/source/common/unicode/bytestriebuilder.h b/icu4c/source/common/unicode/bytestriebuilder.h
index d0a53acbb5c..382f5e0095f 100644
--- a/icu4c/source/common/unicode/bytestriebuilder.h
+++ b/icu4c/source/common/unicode/bytestriebuilder.h
@@ -134,21 +134,21 @@ private:
 
     void buildBytes(UStringTrieBuildOption buildOption, UErrorCode &errorCode);
 
-    virtual int32_t getElementStringLength(int32_t i) const;
-    virtual char16_t getElementUnit(int32_t i, int32_t byteIndex) const;
-    virtual int32_t getElementValue(int32_t i) const;
+    virtual int32_t getElementStringLength(int32_t i) const override;
+    virtual char16_t getElementUnit(int32_t i, int32_t byteIndex) const override;
+    virtual int32_t getElementValue(int32_t i) const override;
 
-    virtual int32_t getLimitOfLinearMatch(int32_t first, int32_t last, int32_t byteIndex) const;
+    virtual int32_t getLimitOfLinearMatch(int32_t first, int32_t last, int32_t byteIndex) const override;
 
-    virtual int32_t countElementUnits(int32_t start, int32_t limit, int32_t byteIndex) const;
-    virtual int32_t skipElementsBySomeUnits(int32_t i, int32_t byteIndex, int32_t count) const;
-    virtual int32_t indexOfElementWithNextUnit(int32_t i, int32_t byteIndex, char16_t byte) const;
+    virtual int32_t countElementUnits(int32_t start, int32_t limit, int32_t byteIndex) const override;
+    virtual int32_t skipElementsBySomeUnits(int32_t i, int32_t byteIndex, int32_t count) const override;
+    virtual int32_t indexOfElementWithNextUnit(int32_t i, int32_t byteIndex, char16_t byte) const override;
 
-    virtual UBool matchNodesCanHaveValues() const { return false; }
+    virtual UBool matchNodesCanHaveValues() const override { return false; }
 
-    virtual int32_t getMaxBranchLinearSubNodeLength() const { return BytesTrie::kMaxBranchLinearSubNodeLength; }
-    virtual int32_t getMinLinearMatch() const { return BytesTrie::kMinLinearMatch; }
-    virtual int32_t getMaxLinearMatchLength() const { return BytesTrie::kMaxLinearMatchLength; }
+    virtual int32_t getMaxBranchLinearSubNodeLength() const override { return BytesTrie::kMaxBranchLinearSubNodeLength; }
+    virtual int32_t getMinLinearMatch() const override { return BytesTrie::kMinLinearMatch; }
+    virtual int32_t getMaxLinearMatchLength() const override { return BytesTrie::kMaxLinearMatchLength; }
 
     /**
      * @internal (private)
@@ -156,22 +156,22 @@ private:
     class BTLinearMatchNode : public LinearMatchNode {
     public:
         BTLinearMatchNode(const char *units, int32_t len, Node *nextNode);
-        virtual bool operator==(const Node &other) const;
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual void write(StringTrieBuilder &builder) override;
     private:
         const char *s;
     };
     
     virtual Node *createLinearMatchNode(int32_t i, int32_t byteIndex, int32_t length,
-                                        Node *nextNode) const;
+                                        Node *nextNode) const override;
 
     UBool ensureCapacity(int32_t length);
-    virtual int32_t write(int32_t byte);
+    virtual int32_t write(int32_t byte) override;
     int32_t write(const char *b, int32_t length);
-    virtual int32_t writeElementUnits(int32_t i, int32_t byteIndex, int32_t length);
-    virtual int32_t writeValueAndFinal(int32_t i, UBool isFinal);
-    virtual int32_t writeValueAndType(UBool hasValue, int32_t value, int32_t node);
-    virtual int32_t writeDeltaTo(int32_t jumpTarget);
+    virtual int32_t writeElementUnits(int32_t i, int32_t byteIndex, int32_t length) override;
+    virtual int32_t writeValueAndFinal(int32_t i, UBool isFinal) override;
+    virtual int32_t writeValueAndType(UBool hasValue, int32_t value, int32_t node) override;
+    virtual int32_t writeDeltaTo(int32_t jumpTarget) override;
     static int32_t internalEncodeDelta(int32_t i, char intBytes[]);
 
     CharString *strings;  // Pointer not object so we need not #include internal charstr.h.
diff --git a/icu4c/source/common/unicode/caniter.h b/icu4c/source/common/unicode/caniter.h
index 205eaaa8316..6e57ef5e3ba 100644
--- a/icu4c/source/common/unicode/caniter.h
+++ b/icu4c/source/common/unicode/caniter.h
@@ -145,7 +145,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     // ===================== PRIVATES ==============================
diff --git a/icu4c/source/common/unicode/dtintrv.h b/icu4c/source/common/unicode/dtintrv.h
index 5aee5c7a49d..46a94ce3495 100644
--- a/icu4c/source/common/unicode/dtintrv.h
+++ b/icu4c/source/common/unicode/dtintrv.h
@@ -89,7 +89,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 4.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     
     /**
diff --git a/icu4c/source/common/unicode/locid.h b/icu4c/source/common/unicode/locid.h
index af1fbc37b26..2f2b3998a78 100644
--- a/icu4c/source/common/unicode/locid.h
+++ b/icu4c/source/common/unicode/locid.h
@@ -1005,7 +1005,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * A Locale iterator interface similar to a Java Iterator<Locale>.
diff --git a/icu4c/source/common/unicode/normlzr.h b/icu4c/source/common/unicode/normlzr.h
index d071182050f..93661990fec 100644
--- a/icu4c/source/common/unicode/normlzr.h
+++ b/icu4c/source/common/unicode/normlzr.h
@@ -732,7 +732,7 @@ public:
    * @return a UClassID for the actual class.
    * @deprecated ICU 56 Use Normalizer2 instead.
    */
-  virtual UClassID getDynamicClassID() const;
+  virtual UClassID getDynamicClassID() const override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
 private:
diff --git a/icu4c/source/common/unicode/parsepos.h b/icu4c/source/common/unicode/parsepos.h
index 67397e45b0a..73945f5f97b 100644
--- a/icu4c/source/common/unicode/parsepos.h
+++ b/icu4c/source/common/unicode/parsepos.h
@@ -166,7 +166,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     /**
diff --git a/icu4c/source/common/unicode/rbbi.h b/icu4c/source/common/unicode/rbbi.h
index 6108261e963..0ce93819f54 100644
--- a/icu4c/source/common/unicode/rbbi.h
+++ b/icu4c/source/common/unicode/rbbi.h
@@ -260,7 +260,7 @@ public:
      * same class, have the same behavior, and iterate over the same text.
      *  @stable ICU 2.0
      */
-    virtual bool operator==(const BreakIterator& that) const;
+    virtual bool operator==(const BreakIterator& that) const override;
 
     /**
      * Not-equal operator.  If operator== returns true, this returns false,
@@ -281,7 +281,7 @@ public:
      * @return a newly-constructed RuleBasedBreakIterator
      * @stable ICU 2.0
      */
-    virtual RuleBasedBreakIterator* clone() const;
+    virtual RuleBasedBreakIterator* clone() const override;
 
     /**
      * Compute a hash code for this BreakIterator
@@ -326,7 +326,7 @@ public:
      * @return An iterator over the text being analyzed.
      * @stable ICU 2.0
      */
-    virtual  CharacterIterator& getText(void) const;
+    virtual  CharacterIterator& getText(void) const override;
 
 
     /**
@@ -343,7 +343,7 @@ public:
       *           UText was provided, it will always be returned.
       * @stable ICU 3.4
       */
-     virtual UText *getUText(UText *fillIn, UErrorCode &status) const;
+     virtual UText *getUText(UText *fillIn, UErrorCode &status) const override;
 
     /**
      * Set the iterator to analyze a new piece of text.  This function resets
@@ -352,7 +352,7 @@ public:
      * takes ownership of the character iterator.  The caller MUST NOT delete it!
      *  @stable ICU 2.0
      */
-    virtual void adoptText(CharacterIterator* newText);
+    virtual void adoptText(CharacterIterator* newText) override;
 
     /**
      * Set the iterator to analyze a new piece of text.  This function resets
@@ -365,7 +365,7 @@ public:
      * @param newText The text to analyze.
      *  @stable ICU 2.0
      */
-    virtual void setText(const UnicodeString& newText);
+    virtual void setText(const UnicodeString& newText) override;
 
     /**
      * Reset the break iterator to operate over the text represented by
@@ -380,21 +380,21 @@ public:
      * @param status  Receives any error codes.
      * @stable ICU 3.4
      */
-    virtual void  setText(UText *text, UErrorCode &status);
+    virtual void  setText(UText *text, UErrorCode &status) override;
 
     /**
      * Sets the current iteration position to the beginning of the text, position zero.
      * @return The offset of the beginning of the text, zero.
      *  @stable ICU 2.0
      */
-    virtual int32_t first(void);
+    virtual int32_t first(void) override;
 
     /**
      * Sets the current iteration position to the end of the text.
      * @return The text's past-the-end offset.
      *  @stable ICU 2.0
      */
-    virtual int32_t last(void);
+    virtual int32_t last(void) override;
 
     /**
      * Advances the iterator either forward or backward the specified number of steps.
@@ -406,21 +406,21 @@ public:
      * the current one.
      *  @stable ICU 2.0
      */
-    virtual int32_t next(int32_t n);
+    virtual int32_t next(int32_t n) override;
 
     /**
      * Advances the iterator to the next boundary position.
      * @return The position of the first boundary after this one.
      *  @stable ICU 2.0
      */
-    virtual int32_t next(void);
+    virtual int32_t next(void) override;
 
     /**
      * Moves the iterator backwards, to the last boundary preceding this one.
      * @return The position of the last boundary position preceding this one.
      *  @stable ICU 2.0
      */
-    virtual int32_t previous(void);
+    virtual int32_t previous(void) override;
 
     /**
      * Sets the iterator to refer to the first boundary position following
@@ -429,7 +429,7 @@ public:
      * @return The position of the first break after the current position.
      *  @stable ICU 2.0
      */
-    virtual int32_t following(int32_t offset);
+    virtual int32_t following(int32_t offset) override;
 
     /**
      * Sets the iterator to refer to the last boundary position before the
@@ -438,7 +438,7 @@ public:
      * @return The position of the last boundary before the starting position.
      *  @stable ICU 2.0
      */
-    virtual int32_t preceding(int32_t offset);
+    virtual int32_t preceding(int32_t offset) override;
 
     /**
      * Returns true if the specified position is a boundary position.  As a side
@@ -448,7 +448,7 @@ public:
      * @return True if "offset" is a boundary position.
      *  @stable ICU 2.0
      */
-    virtual UBool isBoundary(int32_t offset);
+    virtual UBool isBoundary(int32_t offset) override;
 
     /**
      * Returns the current iteration position. Note that UBRK_DONE is never
@@ -458,7 +458,7 @@ public:
      * @return The current iteration position.
      * @stable ICU 2.0
      */
-    virtual int32_t current(void) const;
+    virtual int32_t current(void) const override;
 
 
     /**
@@ -492,7 +492,7 @@ public:
      * @see UWordBreak
      * @stable ICU 2.2
      */
-    virtual int32_t getRuleStatus() const;
+    virtual int32_t getRuleStatus() const override;
 
    /**
     * Get the status (tag) values from the break rule(s) that determined the boundary
@@ -517,7 +517,7 @@ public:
     * @see getRuleStatus
     * @stable ICU 3.0
     */
-    virtual int32_t getRuleStatusVec(int32_t *fillInVec, int32_t capacity, UErrorCode &status);
+    virtual int32_t getRuleStatusVec(int32_t *fillInVec, int32_t capacity, UErrorCode &status) override;
 
     /**
      * Returns a unique class ID POLYMORPHICALLY.  Pure virtual override.
@@ -530,7 +530,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Returns the class ID for this class.  This is useful only for
@@ -574,7 +574,7 @@ public:
      */
     virtual RuleBasedBreakIterator *createBufferClone(void *stackBuffer,
                                                       int32_t &BufferSize,
-                                                      UErrorCode &status);
+                                                      UErrorCode &status) override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -621,7 +621,7 @@ public:
      *
      * @stable ICU 49
      */
-    virtual RuleBasedBreakIterator &refreshInputText(UText *input, UErrorCode &status);
+    virtual RuleBasedBreakIterator &refreshInputText(UText *input, UErrorCode &status) override;
 
 
 private:
diff --git a/icu4c/source/common/unicode/resbund.h b/icu4c/source/common/unicode/resbund.h
index 1cae05c376c..74c8b7ba57d 100644
--- a/icu4c/source/common/unicode/resbund.h
+++ b/icu4c/source/common/unicode/resbund.h
@@ -474,7 +474,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/common/unicode/schriter.h b/icu4c/source/common/unicode/schriter.h
index b7a3e366f38..9dac0999677 100644
--- a/icu4c/source/common/unicode/schriter.h
+++ b/icu4c/source/common/unicode/schriter.h
@@ -124,7 +124,7 @@ public:
    * same string and are pointing at the same character.
    * @stable ICU 2.0
    */
-  virtual bool           operator==(const ForwardCharacterIterator& that) const;
+  virtual bool           operator==(const ForwardCharacterIterator& that) const override;
 
   /**
    * Returns a new StringCharacterIterator referring to the same
@@ -133,7 +133,7 @@ public:
    * @return the newly cloned object.
    * @stable ICU 2.0
    */
-  virtual StringCharacterIterator* clone() const;
+  virtual StringCharacterIterator* clone() const override;
 
   /**
    * Sets the iterator to iterate over the provided string.
@@ -149,14 +149,14 @@ public:
    * @param result Receives a copy of the text under iteration.
    * @stable ICU 2.0
    */
-  virtual void            getText(UnicodeString& result);
+  virtual void            getText(UnicodeString& result) override;
 
   /**
    * Return a class ID for this object (not really public)
    * @return a class ID for this object.
    * @stable ICU 2.0
    */
-  virtual UClassID         getDynamicClassID(void) const;
+  virtual UClassID         getDynamicClassID(void) const override;
 
   /**
    * Return a class ID for this class (not really public)
diff --git a/icu4c/source/common/unicode/stringtriebuilder.h b/icu4c/source/common/unicode/stringtriebuilder.h
index 9ef2a437d48..b7a9b23d22b 100644
--- a/icu4c/source/common/unicode/stringtriebuilder.h
+++ b/icu4c/source/common/unicode/stringtriebuilder.h
@@ -265,8 +265,8 @@ protected:
     class FinalValueNode : public Node {
     public:
         FinalValueNode(int32_t v) : Node(0x111111u*37u+v), value(v) {}
-        virtual bool operator==(const Node &other) const;
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual void write(StringTrieBuilder &builder) override;
     protected:
         int32_t value;
     };
@@ -280,7 +280,7 @@ protected:
     class ValueNode : public Node {
     public:
         ValueNode(int32_t initialHash) : Node(initialHash), hasValue(false), value(0) {}
-        virtual bool operator==(const Node &other) const;
+        virtual bool operator==(const Node &other) const override;
         void setValue(int32_t v) {
             hasValue=true;
             value=v;
@@ -299,9 +299,9 @@ protected:
     public:
         IntermediateValueNode(int32_t v, Node *nextNode)
                 : ValueNode(0x222222u*37u+hashCode(nextNode)), next(nextNode) { setValue(v); }
-        virtual bool operator==(const Node &other) const;
-        virtual int32_t markRightEdgesFirst(int32_t edgeNumber);
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual int32_t markRightEdgesFirst(int32_t edgeNumber) override;
+        virtual void write(StringTrieBuilder &builder) override;
     protected:
         Node *next;
     };
@@ -317,8 +317,8 @@ protected:
         LinearMatchNode(int32_t len, Node *nextNode)
                 : ValueNode((0x333333u*37u+len)*37u+hashCode(nextNode)),
                   length(len), next(nextNode) {}
-        virtual bool operator==(const Node &other) const;
-        virtual int32_t markRightEdgesFirst(int32_t edgeNumber);
+        virtual bool operator==(const Node &other) const override;
+        virtual int32_t markRightEdgesFirst(int32_t edgeNumber) override;
     protected:
         int32_t length;
         Node *next;
@@ -341,9 +341,9 @@ protected:
     class ListBranchNode : public BranchNode {
     public:
         ListBranchNode() : BranchNode(0x444444), length(0) {}
-        virtual bool operator==(const Node &other) const;
-        virtual int32_t markRightEdgesFirst(int32_t edgeNumber);
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual int32_t markRightEdgesFirst(int32_t edgeNumber) override;
+        virtual void write(StringTrieBuilder &builder) override;
         // Adds a unit with a final value.
         void add(int32_t c, int32_t value) {
             units[length]=(char16_t)c;
@@ -376,9 +376,9 @@ protected:
                 : BranchNode(((0x555555u*37u+middleUnit)*37u+
                               hashCode(lessThanNode))*37u+hashCode(greaterOrEqualNode)),
                   unit(middleUnit), lessThan(lessThanNode), greaterOrEqual(greaterOrEqualNode) {}
-        virtual bool operator==(const Node &other) const;
-        virtual int32_t markRightEdgesFirst(int32_t edgeNumber);
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual int32_t markRightEdgesFirst(int32_t edgeNumber) override;
+        virtual void write(StringTrieBuilder &builder) override;
     protected:
         char16_t unit;
         Node *lessThan;
@@ -392,9 +392,9 @@ protected:
         BranchHeadNode(int32_t len, Node *subNode)
                 : ValueNode((0x666666u*37u+len)*37u+hashCode(subNode)),
                   length(len), next(subNode) {}
-        virtual bool operator==(const Node &other) const;
-        virtual int32_t markRightEdgesFirst(int32_t edgeNumber);
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual int32_t markRightEdgesFirst(int32_t edgeNumber) override;
+        virtual void write(StringTrieBuilder &builder) override;
     protected:
         int32_t length;
         Node *next;  // A branch sub-node.
diff --git a/icu4c/source/common/unicode/ucharstriebuilder.h b/icu4c/source/common/unicode/ucharstriebuilder.h
index 4cf3dcdddff..e0cb0acdc53 100644
--- a/icu4c/source/common/unicode/ucharstriebuilder.h
+++ b/icu4c/source/common/unicode/ucharstriebuilder.h
@@ -138,41 +138,41 @@ private:
 
     void buildUChars(UStringTrieBuildOption buildOption, UErrorCode &errorCode);
 
-    virtual int32_t getElementStringLength(int32_t i) const;
-    virtual char16_t getElementUnit(int32_t i, int32_t unitIndex) const;
-    virtual int32_t getElementValue(int32_t i) const;
+    virtual int32_t getElementStringLength(int32_t i) const override;
+    virtual char16_t getElementUnit(int32_t i, int32_t unitIndex) const override;
+    virtual int32_t getElementValue(int32_t i) const override;
 
-    virtual int32_t getLimitOfLinearMatch(int32_t first, int32_t last, int32_t unitIndex) const;
+    virtual int32_t getLimitOfLinearMatch(int32_t first, int32_t last, int32_t unitIndex) const override;
 
-    virtual int32_t countElementUnits(int32_t start, int32_t limit, int32_t unitIndex) const;
-    virtual int32_t skipElementsBySomeUnits(int32_t i, int32_t unitIndex, int32_t count) const;
-    virtual int32_t indexOfElementWithNextUnit(int32_t i, int32_t unitIndex, char16_t unit) const;
+    virtual int32_t countElementUnits(int32_t start, int32_t limit, int32_t unitIndex) const override;
+    virtual int32_t skipElementsBySomeUnits(int32_t i, int32_t unitIndex, int32_t count) const override;
+    virtual int32_t indexOfElementWithNextUnit(int32_t i, int32_t unitIndex, char16_t unit) const override;
 
-    virtual UBool matchNodesCanHaveValues() const { return true; }
+    virtual UBool matchNodesCanHaveValues() const override { return true; }
 
-    virtual int32_t getMaxBranchLinearSubNodeLength() const { return UCharsTrie::kMaxBranchLinearSubNodeLength; }
-    virtual int32_t getMinLinearMatch() const { return UCharsTrie::kMinLinearMatch; }
-    virtual int32_t getMaxLinearMatchLength() const { return UCharsTrie::kMaxLinearMatchLength; }
+    virtual int32_t getMaxBranchLinearSubNodeLength() const override { return UCharsTrie::kMaxBranchLinearSubNodeLength; }
+    virtual int32_t getMinLinearMatch() const override { return UCharsTrie::kMinLinearMatch; }
+    virtual int32_t getMaxLinearMatchLength() const override { return UCharsTrie::kMaxLinearMatchLength; }
 
     class UCTLinearMatchNode : public LinearMatchNode {
     public:
         UCTLinearMatchNode(const char16_t *units, int32_t len, Node *nextNode);
-        virtual bool operator==(const Node &other) const;
-        virtual void write(StringTrieBuilder &builder);
+        virtual bool operator==(const Node &other) const override;
+        virtual void write(StringTrieBuilder &builder) override;
     private:
         const char16_t *s;
     };
 
     virtual Node *createLinearMatchNode(int32_t i, int32_t unitIndex, int32_t length,
-                                        Node *nextNode) const;
+                                        Node *nextNode) const override;
 
     UBool ensureCapacity(int32_t length);
-    virtual int32_t write(int32_t unit);
+    virtual int32_t write(int32_t unit) override;
     int32_t write(const char16_t *s, int32_t length);
-    virtual int32_t writeElementUnits(int32_t i, int32_t unitIndex, int32_t length);
-    virtual int32_t writeValueAndFinal(int32_t i, UBool isFinal);
-    virtual int32_t writeValueAndType(UBool hasValue, int32_t value, int32_t node);
-    virtual int32_t writeDeltaTo(int32_t jumpTarget);
+    virtual int32_t writeElementUnits(int32_t i, int32_t unitIndex, int32_t length) override;
+    virtual int32_t writeValueAndFinal(int32_t i, UBool isFinal) override;
+    virtual int32_t writeValueAndType(UBool hasValue, int32_t value, int32_t node) override;
+    virtual int32_t writeDeltaTo(int32_t jumpTarget) override;
 
     UnicodeString strings;
     UCharsTrieElement *elements;
diff --git a/icu4c/source/common/unicode/uchriter.h b/icu4c/source/common/unicode/uchriter.h
index 76c73546444..9fae5e7de08 100644
--- a/icu4c/source/common/unicode/uchriter.h
+++ b/icu4c/source/common/unicode/uchriter.h
@@ -119,14 +119,14 @@ public:
    * same string and are pointing at the same character.
    * @stable ICU 2.0
    */
-  virtual bool           operator==(const ForwardCharacterIterator& that) const;
+  virtual bool           operator==(const ForwardCharacterIterator& that) const override;
 
   /**
    * Generates a hash code for this iterator.
    * @return the hash code.
    * @stable ICU 2.0
    */
-  virtual int32_t         hashCode(void) const;
+  virtual int32_t         hashCode(void) const override;
 
   /**
    * Returns a new UCharCharacterIterator referring to the same
@@ -135,7 +135,7 @@ public:
    * @return the CharacterIterator newly created
    * @stable ICU 2.0
    */
-  virtual UCharCharacterIterator* clone() const;
+  virtual UCharCharacterIterator* clone() const override;
 
   /**
    * Sets the iterator to refer to the first code unit in its
@@ -144,7 +144,7 @@ public:
    * @return the first code unit in its iteration range.
    * @stable ICU 2.0
    */
-  virtual char16_t         first(void);
+  virtual char16_t         first(void) override;
 
   /**
    * Sets the iterator to refer to the first code unit in its
@@ -154,7 +154,7 @@ public:
    * @return the first code unit in its iteration range
    * @stable ICU 2.0
    */
-  virtual char16_t         firstPostInc(void);
+  virtual char16_t         firstPostInc(void) override;
 
   /**
    * Sets the iterator to refer to the first code point in its
@@ -165,7 +165,7 @@ public:
    * @return the first code point in its iteration range
    * @stable ICU 2.0
    */
-  virtual UChar32       first32(void);
+  virtual UChar32       first32(void) override;
 
   /**
    * Sets the iterator to refer to the first code point in its
@@ -175,7 +175,7 @@ public:
    * @return the first code point in its iteration range.
    * @stable ICU 2.0
    */
-  virtual UChar32       first32PostInc(void);
+  virtual UChar32       first32PostInc(void) override;
 
   /**
    * Sets the iterator to refer to the last code unit in its
@@ -184,7 +184,7 @@ public:
    * @return the last code unit in its iteration range.
    * @stable ICU 2.0
    */
-  virtual char16_t         last(void);
+  virtual char16_t         last(void) override;
 
   /**
    * Sets the iterator to refer to the last code point in its
@@ -193,7 +193,7 @@ public:
    * @return the last code point in its iteration range.
    * @stable ICU 2.0
    */
-  virtual UChar32       last32(void);
+  virtual UChar32       last32(void) override;
 
   /**
    * Sets the iterator to refer to the "position"-th code unit
@@ -203,7 +203,7 @@ public:
    * @return the code unit
    * @stable ICU 2.0
    */
-  virtual char16_t         setIndex(int32_t position);
+  virtual char16_t         setIndex(int32_t position) override;
 
   /**
    * Sets the iterator to refer to the beginning of the code point
@@ -216,21 +216,21 @@ public:
    * @return the code unit
    * @stable ICU 2.0
    */
-  virtual UChar32       setIndex32(int32_t position);
+  virtual UChar32       setIndex32(int32_t position) override;
 
   /**
    * Returns the code unit the iterator currently refers to.
    * @return the code unit the iterator currently refers to.
    * @stable ICU 2.0
    */
-  virtual char16_t         current(void) const;
+  virtual char16_t         current(void) const override;
 
   /**
    * Returns the code point the iterator currently refers to.
    * @return the code point the iterator currently refers to.
    * @stable ICU 2.0
    */
-  virtual UChar32       current32(void) const;
+  virtual UChar32       current32(void) const override;
 
   /**
    * Advances to the next code unit in the iteration range (toward
@@ -239,7 +239,7 @@ public:
    * @return the next code unit in the iteration range.
    * @stable ICU 2.0
    */
-  virtual char16_t         next(void);
+  virtual char16_t         next(void) override;
 
   /**
    * Gets the current code unit for returning and advances to the next code unit
@@ -249,7 +249,7 @@ public:
    * @return the current code unit.
    * @stable ICU 2.0
    */
-  virtual char16_t         nextPostInc(void);
+  virtual char16_t         nextPostInc(void) override;
 
   /**
    * Advances to the next code point in the iteration range (toward
@@ -261,7 +261,7 @@ public:
    * @return the next code point in the iteration range.
    * @stable ICU 2.0
    */
-  virtual UChar32       next32(void);
+  virtual UChar32       next32(void) override;
 
   /**
    * Gets the current code point for returning and advances to the next code point
@@ -271,7 +271,7 @@ public:
    * @return the current point.
    * @stable ICU 2.0
    */
-  virtual UChar32       next32PostInc(void);
+  virtual UChar32       next32PostInc(void) override;
 
   /**
    * Returns false if there are no more code units or code points
@@ -282,7 +282,7 @@ public:
    * at or after the current position in the iteration range.
    * @stable ICU 2.0
    */
-  virtual UBool        hasNext();
+  virtual UBool        hasNext() override;
 
   /**
    * Advances to the previous code unit in the iteration range (toward
@@ -291,7 +291,7 @@ public:
    * @return the previous code unit in the iteration range.
    * @stable ICU 2.0
    */
-  virtual char16_t         previous(void);
+  virtual char16_t         previous(void) override;
 
   /**
    * Advances to the previous code point in the iteration range (toward
@@ -300,7 +300,7 @@ public:
    * @return the previous code point in the iteration range.
    * @stable ICU 2.0
    */
-  virtual UChar32       previous32(void);
+  virtual UChar32       previous32(void) override;
 
   /**
    * Returns false if there are no more code units or code points
@@ -311,7 +311,7 @@ public:
    * before the current position in the iteration range.
    * @stable ICU 2.0
    */
-  virtual UBool        hasPrevious();
+  virtual UBool        hasPrevious() override;
 
   /**
    * Moves the current position relative to the start or end of the
@@ -324,7 +324,7 @@ public:
    * @return the new position
    * @stable ICU 2.0
    */
-  virtual int32_t      move(int32_t delta, EOrigin origin);
+  virtual int32_t      move(int32_t delta, EOrigin origin) override;
 
   /**
    * Moves the current position relative to the start or end of the
@@ -341,7 +341,7 @@ public:
    // One of the system headers right now is sometimes defining a conflicting macro we don't use
 #undef move32
 #endif
-  virtual int32_t      move32(int32_t delta, EOrigin origin);
+  virtual int32_t      move32(int32_t delta, EOrigin origin) override;
 
   /**
    * Sets the iterator to iterate over a new range of text
@@ -356,7 +356,7 @@ public:
    * @param result Receives a copy of the text under iteration.
    * @stable ICU 2.0
    */
-  virtual void            getText(UnicodeString& result);
+  virtual void            getText(UnicodeString& result) override;
 
   /**
    * Return a class ID for this class (not really public)
@@ -370,7 +370,7 @@ public:
    * @return a class ID for this object.
    * @stable ICU 2.0
    */
-  virtual UClassID        getDynamicClassID(void) const;
+  virtual UClassID        getDynamicClassID(void) const override;
 
 protected:
   /**
diff --git a/icu4c/source/common/unicode/uniset.h b/icu4c/source/common/unicode/uniset.h
index dc3e0b24594..d0cc86e02d2 100644
--- a/icu4c/source/common/unicode/uniset.h
+++ b/icu4c/source/common/unicode/uniset.h
@@ -502,7 +502,7 @@ public:
      * @see cloneAsThawed
      * @stable ICU 2.0
      */
-    virtual UnicodeSet* clone() const;
+    virtual UnicodeSet* clone() const override;
 
     /**
      * Returns the hash code value for this set.
@@ -704,7 +704,7 @@ public:
      * @stable ICU 2.0
      */
     virtual UnicodeString& toPattern(UnicodeString& result,
-                                     UBool escapeUnprintable = false) const;
+                                     UBool escapeUnprintable = false) const override;
 
     /**
      * Modifies this set to contain those code points which have the given value
@@ -802,7 +802,7 @@ public:
      * @return true if the test condition is met
      * @stable ICU 2.0
      */
-    virtual UBool contains(UChar32 c) const;
+    virtual UBool contains(UChar32 c) const override;
 
     /**
      * Returns true if this set contains every character
@@ -1011,7 +1011,7 @@ public:
     virtual UMatchDegree matches(const Replaceable& text,
                          int32_t& offset,
                          int32_t limit,
-                         UBool incremental);
+                         UBool incremental) override;
 
 private:
     /**
@@ -1060,7 +1060,7 @@ public:
      * @param toUnionTo the set into which to union the source characters
      * @stable ICU 2.4
      */
-    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const override;
 
     /**
      * Returns the index of the given character within this set, where
@@ -1521,7 +1521,7 @@ public:
      * different class IDs.
      * @stable ICU 2.4
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
 private:
 
@@ -1542,7 +1542,7 @@ private:
      * is the given value.  This is used by <tt>RuleBasedTransliterator</tt> for
      * indexing.
      */
-    virtual UBool matchesIndexValue(uint8_t v) const;
+    virtual UBool matchesIndexValue(uint8_t v) const override;
 
 private:
     friend class RBBIRuleScanner;
diff --git a/icu4c/source/common/unicode/unistr.h b/icu4c/source/common/unicode/unistr.h
index 936a631fcb6..b3c99481079 100644
--- a/icu4c/source/common/unicode/unistr.h
+++ b/icu4c/source/common/unicode/unistr.h
@@ -1526,7 +1526,7 @@ public:
    */
   virtual void extractBetween(int32_t start,
               int32_t limit,
-              UnicodeString& target) const;
+              UnicodeString& target) const override;
 
   /**
    * Copy the characters in the range
@@ -2456,14 +2456,14 @@ public:
    */
   virtual void handleReplaceBetween(int32_t start,
                                     int32_t limit,
-                                    const UnicodeString& text);
+                                    const UnicodeString& text) override;
 
   /**
    * Replaceable API
    * @return true if it has MetaData
    * @stable ICU 2.4
    */
-  virtual UBool hasMetaData() const;
+  virtual UBool hasMetaData() const override;
 
   /**
    * Copy a substring of this object, retaining attribute (out-of-band)
@@ -2478,7 +2478,7 @@ public:
    * dest >= limit`.
    * @stable ICU 2.0
    */
-  virtual void copy(int32_t start, int32_t limit, int32_t dest);
+  virtual void copy(int32_t start, int32_t limit, int32_t dest) override;
 
   /* Search and replace operations */
 
@@ -3338,7 +3338,7 @@ public:
    * @see getDynamicClassID
    * @stable ICU 2.6
    */
-  virtual UnicodeString *clone() const;
+  virtual UnicodeString *clone() const override;
 
   /** Destructor.
    * @stable ICU 2.0
@@ -3444,7 +3444,7 @@ public:
    *
    * @stable ICU 2.2
    */
-  virtual UClassID getDynamicClassID() const;
+  virtual UClassID getDynamicClassID() const override;
 
   //========================================
   // Implementation methods
@@ -3455,21 +3455,21 @@ protected:
    * Implement Replaceable::getLength() (see jitterbug 1027).
    * @stable ICU 2.4
    */
-  virtual int32_t getLength() const;
+  virtual int32_t getLength() const override;
 
   /**
    * The change in Replaceable to use virtual getCharAt() allows
    * UnicodeString::charAt() to be inline again (see jitterbug 709).
    * @stable ICU 2.4
    */
-  virtual char16_t getCharAt(int32_t offset) const;
+  virtual char16_t getCharAt(int32_t offset) const override;
 
   /**
    * The change in Replaceable to use virtual getChar32At() allows
    * UnicodeString::char32At() to be inline again (see jitterbug 709).
    * @stable ICU 2.4
    */
-  virtual UChar32 getChar32At(int32_t offset) const;
+  virtual UChar32 getChar32At(int32_t offset) const override;
 
 private:
   // For char* constructors. Could be made public.
diff --git a/icu4c/source/common/unicode/usetiter.h b/icu4c/source/common/unicode/usetiter.h
index 95226d97b15..5834c308e41 100644
--- a/icu4c/source/common/unicode/usetiter.h
+++ b/icu4c/source/common/unicode/usetiter.h
@@ -255,7 +255,7 @@ class U_COMMON_API UnicodeSetIterator U_FINAL : public UObject {
      *
      * @stable ICU 2.4
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     // ======================= PRIVATES ===========================
 
diff --git a/icu4c/source/common/unifiedcache.h b/icu4c/source/common/unifiedcache.h
index f22481b348c..07a734b8bd5 100644
--- a/icu4c/source/common/unifiedcache.h
+++ b/icu4c/source/common/unifiedcache.h
@@ -107,7 +107,7 @@ class CacheKey : public CacheKeyBase {
    /**
     * The template parameter, T, determines the hash code returned.
     */
-   virtual int32_t hashCode() const {
+   virtual int32_t hashCode() const override {
        const char *s = typeid(T).name();
        return ustr_hashCharsN(s, static_cast<int32_t>(uprv_strlen(s)));
    }
@@ -115,7 +115,7 @@ class CacheKey : public CacheKeyBase {
    /**
     * Use the value type, T,  as the description.
     */
-   virtual char *writeDescription(char *buffer, int32_t bufLen) const {
+   virtual char *writeDescription(char *buffer, int32_t bufLen) const override {
        const char *s = typeid(T).name();
        uprv_strncpy(buffer, s, bufLen);
        buffer[bufLen - 1] = 0;
@@ -126,7 +126,7 @@ class CacheKey : public CacheKeyBase {
    /**
     * Two objects are equal if they are of the same type.
     */
-   virtual bool equals(const CacheKeyBase &other) const {
+   virtual bool equals(const CacheKeyBase &other) const override {
        return this == &other || typeid(*this) == typeid(other);
    }
 };
@@ -139,7 +139,7 @@ template<typename T>
 class LocaleCacheKey : public CacheKey<T> {
  protected:
    Locale   fLoc;
-   virtual bool equals(const CacheKeyBase &other) const {
+   virtual bool equals(const CacheKeyBase &other) const override {
        if (!CacheKey<T>::equals(other)) {
            return false;
        }
@@ -152,21 +152,21 @@ class LocaleCacheKey : public CacheKey<T> {
    LocaleCacheKey(const LocaleCacheKey<T> &other)
            : CacheKey<T>(other), fLoc(other.fLoc) { }
    virtual ~LocaleCacheKey() { }
-   virtual int32_t hashCode() const {
+   virtual int32_t hashCode() const override {
        return (int32_t)(37u * (uint32_t)CacheKey<T>::hashCode() + (uint32_t)fLoc.hashCode());
    }
    inline bool operator == (const LocaleCacheKey<T> &other) const {
        return fLoc == other.fLoc;
    }
-   virtual CacheKeyBase *clone() const {
+   virtual CacheKeyBase *clone() const override {
        return new LocaleCacheKey<T>(*this);
    }
    virtual const T *createObject(
-           const void *creationContext, UErrorCode &status) const;
+           const void *creationContext, UErrorCode &status) const override;
    /**
     * Use the locale id as the description.
     */
-   virtual char *writeDescription(char *buffer, int32_t bufLen) const {
+   virtual char *writeDescription(char *buffer, int32_t bufLen) const override {
        const char *s = fLoc.getName();
        uprv_strncpy(buffer, s, bufLen);
        buffer[bufLen - 1] = 0;
@@ -341,7 +341,7 @@ class U_COMMON_API UnifiedCache : public UnifiedCacheBase {
     */
    int32_t unusedCount() const;
 
-   virtual void handleUnreferencedObject() const;
+   virtual void handleUnreferencedObject() const override;
    virtual ~UnifiedCache();
    
  private:
diff --git a/icu4c/source/common/uresdata.h b/icu4c/source/common/uresdata.h
index 8f6ba64dcd6..b8a3adba634 100644
--- a/icu4c/source/common/uresdata.h
+++ b/icu4c/source/common/uresdata.h
@@ -535,21 +535,21 @@ public:
     const ResourceData &getData() const { return *pResData; }
     UResourceDataEntry *getValidLocaleDataEntry() const { return validLocaleDataEntry; }
     Resource getResource() const { return res; }
-    virtual UResType getType() const;
-    virtual const UChar *getString(int32_t &length, UErrorCode &errorCode) const;
-    virtual const UChar *getAliasString(int32_t &length, UErrorCode &errorCode) const;
-    virtual int32_t getInt(UErrorCode &errorCode) const;
-    virtual uint32_t getUInt(UErrorCode &errorCode) const;
-    virtual const int32_t *getIntVector(int32_t &length, UErrorCode &errorCode) const;
-    virtual const uint8_t *getBinary(int32_t &length, UErrorCode &errorCode) const;
-    virtual ResourceArray getArray(UErrorCode &errorCode) const;
-    virtual ResourceTable getTable(UErrorCode &errorCode) const;
-    virtual UBool isNoInheritanceMarker() const;
+    virtual UResType getType() const override;
+    virtual const UChar *getString(int32_t &length, UErrorCode &errorCode) const override;
+    virtual const UChar *getAliasString(int32_t &length, UErrorCode &errorCode) const override;
+    virtual int32_t getInt(UErrorCode &errorCode) const override;
+    virtual uint32_t getUInt(UErrorCode &errorCode) const override;
+    virtual const int32_t *getIntVector(int32_t &length, UErrorCode &errorCode) const override;
+    virtual const uint8_t *getBinary(int32_t &length, UErrorCode &errorCode) const override;
+    virtual ResourceArray getArray(UErrorCode &errorCode) const override;
+    virtual ResourceTable getTable(UErrorCode &errorCode) const override;
+    virtual UBool isNoInheritanceMarker() const override;
     virtual int32_t getStringArray(UnicodeString *dest, int32_t capacity,
-                                   UErrorCode &errorCode) const;
+                                   UErrorCode &errorCode) const override;
     virtual int32_t getStringArrayOrStringAsArray(UnicodeString *dest, int32_t capacity,
-                                                  UErrorCode &errorCode) const;
-    virtual UnicodeString getStringOrFirstOfArray(UErrorCode &errorCode) const;
+                                                  UErrorCode &errorCode) const override;
+    virtual UnicodeString getStringOrFirstOfArray(UErrorCode &errorCode) const override;
 
 private:
     const ResourceData *pResData;
diff --git a/icu4c/source/common/ustrenum.h b/icu4c/source/common/ustrenum.h
index a82162e2bde..3703dedb97d 100644
--- a/icu4c/source/common/ustrenum.h
+++ b/icu4c/source/common/ustrenum.h
@@ -47,9 +47,9 @@ public:
      * @param status the error code.
      * @return number of elements in the iterator.
      */
-    virtual int32_t count(UErrorCode& status) const;
+    virtual int32_t count(UErrorCode& status) const override;
 
-    virtual const char* next(int32_t *resultLength, UErrorCode& status);
+    virtual const char* next(int32_t *resultLength, UErrorCode& status) override;
 
     /**
      * Returns the next element a UnicodeString*.  If there are no
@@ -57,18 +57,18 @@ public:
      * @param status the error code.
      * @return a pointer to the string, or NULL.
      */
-    virtual const UnicodeString* snext(UErrorCode& status);
+    virtual const UnicodeString* snext(UErrorCode& status) override;
 
     /**
      * Resets the iterator.
      * @param status the error code.
      */
-    virtual void reset(UErrorCode& status);
+    virtual void reset(UErrorCode& status) override;
 
     /**
      * ICU4C "poor man's RTTI", returns a UClassID for the actual ICU class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU4C "poor man's RTTI", returns a UClassID for this ICU class.
diff --git a/icu4c/source/common/uts46.cpp b/icu4c/source/common/uts46.cpp
index f25b4e12f12..6f930703390 100644
--- a/icu4c/source/common/uts46.cpp
+++ b/icu4c/source/common/uts46.cpp
@@ -118,35 +118,35 @@ public:
 
     virtual UnicodeString &
     labelToASCII(const UnicodeString &label, UnicodeString &dest,
-                 IDNAInfo &info, UErrorCode &errorCode) const;
+                 IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual UnicodeString &
     labelToUnicode(const UnicodeString &label, UnicodeString &dest,
-                   IDNAInfo &info, UErrorCode &errorCode) const;
+                   IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual UnicodeString &
     nameToASCII(const UnicodeString &name, UnicodeString &dest,
-                IDNAInfo &info, UErrorCode &errorCode) const;
+                IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual UnicodeString &
     nameToUnicode(const UnicodeString &name, UnicodeString &dest,
-                  IDNAInfo &info, UErrorCode &errorCode) const;
+                  IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual void
     labelToASCII_UTF8(StringPiece label, ByteSink &dest,
-                      IDNAInfo &info, UErrorCode &errorCode) const;
+                      IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual void
     labelToUnicodeUTF8(StringPiece label, ByteSink &dest,
-                       IDNAInfo &info, UErrorCode &errorCode) const;
+                       IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual void
     nameToASCII_UTF8(StringPiece name, ByteSink &dest,
-                     IDNAInfo &info, UErrorCode &errorCode) const;
+                     IDNAInfo &info, UErrorCode &errorCode) const override;
 
     virtual void
     nameToUnicodeUTF8(StringPiece name, ByteSink &dest,
-                      IDNAInfo &info, UErrorCode &errorCode) const;
+                      IDNAInfo &info, UErrorCode &errorCode) const override;
 
 private:
     UnicodeString &
diff --git a/icu4c/source/common/uvector.h b/icu4c/source/common/uvector.h
index ff086308594..66ddcd22f79 100644
--- a/icu4c/source/common/uvector.h
+++ b/icu4c/source/common/uvector.h
@@ -292,7 +292,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     int32_t indexOf(UElement key, int32_t startIndex = 0, int8_t hint = 0) const;
@@ -374,7 +374,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     // Disallow
     UStack(const UStack&) = delete;
diff --git a/icu4c/source/common/uvectr32.h b/icu4c/source/common/uvectr32.h
index bb829ca4a13..f08c2ade765 100644
--- a/icu4c/source/common/uvectr32.h
+++ b/icu4c/source/common/uvectr32.h
@@ -181,7 +181,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     void _init(int32_t initialCapacity, UErrorCode &status);
diff --git a/icu4c/source/common/uvectr64.h b/icu4c/source/common/uvectr64.h
index 3ccca65c571..ba3eca268df 100644
--- a/icu4c/source/common/uvectr64.h
+++ b/icu4c/source/common/uvectr64.h
@@ -170,7 +170,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     void _init(int32_t initialCapacity, UErrorCode &status);
diff --git a/icu4c/source/extra/scrptrun/scrptrun.h b/icu4c/source/extra/scrptrun/scrptrun.h
index ac6bfe327e8..7f033a72834 100644
--- a/icu4c/source/extra/scrptrun/scrptrun.h
+++ b/icu4c/source/extra/scrptrun/scrptrun.h
@@ -62,7 +62,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual inline UClassID getDynamicClassID() const { return getStaticClassID(); }
+    virtual inline UClassID getDynamicClassID() const override { return getStaticClassID(); }
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/anytrans.h b/icu4c/source/i18n/anytrans.h
index 627dee3c814..67ebb2e7d2f 100644
--- a/icu4c/source/i18n/anytrans.h
+++ b/icu4c/source/i18n/anytrans.h
@@ -66,18 +66,18 @@ public:
     /**
      * Transliterator API.
      */
-    virtual AnyTransliterator* clone() const;
+    virtual AnyTransliterator* clone() const override;
 
     /**
      * Implements {@link Transliterator#handleTransliterate}.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& index,
-                                     UBool incremental) const;
+                                     UBool incremental) const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/astro.cpp b/icu4c/source/i18n/astro.cpp
index f6e494de920..6dc463b4ca4 100644
--- a/icu4c/source/i18n/astro.cpp
+++ b/icu4c/source/i18n/astro.cpp
@@ -723,7 +723,7 @@ CalendarAstronomer::AngleFunc::~AngleFunc() {}
 class SunTimeAngleFunc : public CalendarAstronomer::AngleFunc {
 public:
     virtual ~SunTimeAngleFunc();
-    virtual double eval(CalendarAstronomer& a) { return a.getSunLongitude(); }
+    virtual double eval(CalendarAstronomer& a) override { return a.getSunLongitude(); }
 };
 
 SunTimeAngleFunc::~SunTimeAngleFunc() {}
@@ -743,7 +743,7 @@ CalendarAstronomer::CoordFunc::~CoordFunc() {}
 class RiseSetCoordFunc : public CalendarAstronomer::CoordFunc {
 public:
     virtual ~RiseSetCoordFunc();
-    virtual void eval(CalendarAstronomer::Equatorial& result, CalendarAstronomer&a) {  a.getSunPosition(result); }
+    virtual void eval(CalendarAstronomer::Equatorial& result, CalendarAstronomer& a) override { a.getSunPosition(result); }
 };
 
 RiseSetCoordFunc::~RiseSetCoordFunc() {}
@@ -1225,7 +1225,7 @@ const CalendarAstronomer::MoonAge CalendarAstronomer::FULL_MOON() {
 class MoonTimeAngleFunc : public CalendarAstronomer::AngleFunc {
 public:
     virtual ~MoonTimeAngleFunc();
-    virtual double eval(CalendarAstronomer&a) { return a.getMoonAge(); }
+    virtual double eval(CalendarAstronomer& a) override { return a.getMoonAge(); }
 };
 
 MoonTimeAngleFunc::~MoonTimeAngleFunc() {}
@@ -1271,7 +1271,7 @@ UDate CalendarAstronomer::getMoonTime(const CalendarAstronomer::MoonAge& desired
 class MoonRiseSetCoordFunc : public CalendarAstronomer::CoordFunc {
 public:
     virtual ~MoonRiseSetCoordFunc();
-    virtual void eval(CalendarAstronomer::Equatorial& result, CalendarAstronomer&a) { result = a.getMoonPosition(); }
+    virtual void eval(CalendarAstronomer::Equatorial& result, CalendarAstronomer& a) override { result = a.getMoonPosition(); }
 };
 
 MoonRiseSetCoordFunc::~MoonRiseSetCoordFunc() {}
diff --git a/icu4c/source/i18n/brktrans.h b/icu4c/source/i18n/brktrans.h
index cb3def9e926..5dcc8c50c02 100644
--- a/icu4c/source/i18n/brktrans.h
+++ b/icu4c/source/i18n/brktrans.h
@@ -54,7 +54,7 @@ public:
      * Transliterator API.
      * @return    A copy of the object.
      */
-    virtual BreakTransliterator* clone() const;
+    virtual BreakTransliterator* clone() const override;
 
     virtual const UnicodeString &getInsertion() const;
 
@@ -63,7 +63,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -82,7 +82,7 @@ public:
      *                      pos.contextLimit. Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
  private:
      LocalPointer<BreakIterator> cachedBI;
diff --git a/icu4c/source/i18n/buddhcal.h b/icu4c/source/i18n/buddhcal.h
index 7fc686dcde2..2ef5c524935 100644
--- a/icu4c/source/i18n/buddhcal.h
+++ b/icu4c/source/i18n/buddhcal.h
@@ -94,7 +94,7 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @internal
      */
-    virtual BuddhistCalendar* clone() const;
+    virtual BuddhistCalendar* clone() const override;
 
 public:
     /**
@@ -107,7 +107,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @internal
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
@@ -128,7 +128,7 @@ public:
      * @return calendar type
      * @internal
      */
-    virtual const char * getType() const;
+    virtual const char * getType() const override;
 
 private:
     BuddhistCalendar(); // default constructor not implemented
@@ -142,13 +142,13 @@ private:
      * @return the extended year
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
     /**
      * Subclasses may override this method to compute several fields
      * specific to each calendar system.  
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode& status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode& status) override;
     /**
      * Subclass API for defining limits of different types.
      * @param field one of the field numbers
@@ -156,7 +156,7 @@ private:
      * <code>LEAST_MAXIMUM</code>, or <code>MAXIMUM</code>
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
         /**
      * Return the Julian day number of day before the first day of the
      * given month in the given extended year.  Subclasses should override
@@ -171,26 +171,26 @@ private:
      * @internal
      */
     virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month,
-                                            UBool useMonth) const;
+                                            UBool useMonth) const override;
 
     /**
      * Returns true because the Buddhist Calendar does have a default century
      * @internal
      */
-    virtual UBool haveDefaultCentury() const;
+    virtual UBool haveDefaultCentury() const override;
 
     /**
      * Returns the date of the start of the default century
      * @return start of century - in milliseconds since epoch, 1970
      * @internal
      */
-    virtual UDate defaultCenturyStart() const;
+    virtual UDate defaultCenturyStart() const override;
 
     /**
      * Returns the year in which the default century begins
      * @internal
      */
-    virtual int32_t defaultCenturyStartYear() const;
+    virtual int32_t defaultCenturyStartYear() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/calendar.cpp b/icu4c/source/i18n/calendar.cpp
index 0bf074e5c7c..8405d08d3cf 100644
--- a/icu4c/source/i18n/calendar.cpp
+++ b/icu4c/source/i18n/calendar.cpp
@@ -427,7 +427,7 @@ protected:
     //  return isStandardSupportedKeyword(keyword, status);
     //}
 
-    virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const
+    virtual void updateVisibleIDs(Hashtable& result, UErrorCode& status) const override
     {
         if (U_SUCCESS(status)) {
             for(int32_t i=0;gCalTypes[i] != NULL;i++) {
@@ -439,7 +439,7 @@ protected:
         }
     }
 
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* /*service*/, UErrorCode& status) const {
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* /*service*/, UErrorCode& status) const override {
 #ifdef U_DEBUG_CALSVC
         if(dynamic_cast<const LocaleKey*>(&key) == NULL) {
             fprintf(stderr, "::create - not a LocaleKey!\n");
@@ -485,7 +485,7 @@ public:
     DefaultCalendarFactory() : ICUResourceBundleFactory() { }
     virtual ~DefaultCalendarFactory();
 protected:
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* /*service*/, UErrorCode& status) const  {
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* /*service*/, UErrorCode& status) const override {
 
         LocaleKey &lkey = (LocaleKey&)key;
         Locale loc;
@@ -517,7 +517,7 @@ public:
 
     virtual ~CalendarService();
 
-    virtual UObject* cloneInstance(UObject* instance) const {
+    virtual UObject* cloneInstance(UObject* instance) const override {
         UnicodeString *s = dynamic_cast<UnicodeString *>(instance);
         if(s != NULL) {
             return s->clone();
@@ -530,7 +530,7 @@ public:
         }
     }
 
-    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* /*actualID*/, UErrorCode& status) const {
+    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* /*actualID*/, UErrorCode& status) const override {
         LocaleKey& lkey = (LocaleKey&)key;
         //int32_t kind = lkey.kind();
 
@@ -555,7 +555,7 @@ public:
         return nc;
     }
 
-    virtual UBool isDefault() const {
+    virtual UBool isDefault() const override {
         return countFactories() == 1;
     }
 };
diff --git a/icu4c/source/i18n/cecal.h b/icu4c/source/i18n/cecal.h
index 80dab70f6ff..9ac71f6ba38 100644
--- a/icu4c/source/i18n/cecal.h
+++ b/icu4c/source/i18n/cecal.h
@@ -68,13 +68,13 @@ protected:
      * Return JD of start of given month/extended year
      * @internal
      */
-    virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const;
+    virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const override;
 
     /**
      * Calculate the limit for a specified type of limit and field
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
 
     /**
      * (Overrides Calendar) Return true if the current date for this Calendar is in
@@ -85,13 +85,13 @@ protected:
      *           false, otherwise.
      * @internal
      */
-    virtual UBool inDaylightTime(UErrorCode&) const;
+    virtual UBool inDaylightTime(UErrorCode&) const override;
 
     /**
      * Returns true because Coptic/Ethiopic Calendar does have a default century
      * @internal
      */
-    virtual UBool haveDefaultCentury() const;
+    virtual UBool haveDefaultCentury() const override;
 
 protected:
     /**
diff --git a/icu4c/source/i18n/chnsecal.h b/icu4c/source/i18n/chnsecal.h
index 910afd2f599..61ef2d3cad5 100644
--- a/icu4c/source/i18n/chnsecal.h
+++ b/icu4c/source/i18n/chnsecal.h
@@ -144,7 +144,7 @@ class U_I18N_API ChineseCalendar : public Calendar {
   virtual ~ChineseCalendar();
 
   // clone
-  virtual ChineseCalendar* clone() const;
+  virtual ChineseCalendar* clone() const override;
 
  private:
 
@@ -162,18 +162,18 @@ class U_I18N_API ChineseCalendar : public Calendar {
   //----------------------------------------------------------------------
 
  protected:
-  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
-  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const;
-  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const;
-  virtual int32_t handleGetExtendedYear();
-  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
-  virtual const UFieldResolutionTable* getFieldResolutionTable() const;
+  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
+  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const override;
+  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const override;
+  virtual int32_t handleGetExtendedYear() override;
+  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
+  virtual const UFieldResolutionTable* getFieldResolutionTable() const override;
 
  public:
-  virtual void add(UCalendarDateFields field, int32_t amount, UErrorCode &status);
-  virtual void add(EDateFields field, int32_t amount, UErrorCode &status);
-  virtual void roll(UCalendarDateFields field, int32_t amount, UErrorCode &status);
-  virtual void roll(EDateFields field, int32_t amount, UErrorCode &status);
+  virtual void add(UCalendarDateFields field, int32_t amount, UErrorCode &status) override;
+  virtual void add(EDateFields field, int32_t amount, UErrorCode &status) override;
+  virtual void roll(UCalendarDateFields field, int32_t amount, UErrorCode &status) override;
+  virtual void roll(EDateFields field, int32_t amount, UErrorCode &status) override;
 
   //----------------------------------------------------------------------
   // Internal methods & astronomical calculations
@@ -204,7 +204,7 @@ class U_I18N_API ChineseCalendar : public Calendar {
    *           same class ID. Objects of other classes have different class IDs.
    * @internal
    */
-  virtual UClassID getDynamicClassID(void) const;
+  virtual UClassID getDynamicClassID(void) const override;
 
   /**
    * Return the class ID for this class. This is useful only for comparing to a return
@@ -225,7 +225,7 @@ class U_I18N_API ChineseCalendar : public Calendar {
    * @return calendar type
    * @internal
    */
-  virtual const char * getType() const;
+  virtual const char * getType() const override;
 
 
  protected:
@@ -238,27 +238,27 @@ class U_I18N_API ChineseCalendar : public Calendar {
    *           false, otherwise.
    * @internal
    */
-  virtual UBool inDaylightTime(UErrorCode& status) const;
+  virtual UBool inDaylightTime(UErrorCode& status) const override;
 
 
   /**
    * Returns true because the Islamic Calendar does have a default century
    * @internal
    */
-  virtual UBool haveDefaultCentury() const;
+  virtual UBool haveDefaultCentury() const override;
 
   /**
    * Returns the date of the start of the default century
    * @return start of century - in milliseconds since epoch, 1970
    * @internal
    */
-  virtual UDate defaultCenturyStart() const;
+  virtual UDate defaultCenturyStart() const override;
 
   /**
    * Returns the year in which the default century begins
    * @internal
    */
-  virtual int32_t defaultCenturyStartYear() const;
+  virtual int32_t defaultCenturyStartYear() const override;
 
  private: // default century stuff.
 
diff --git a/icu4c/source/i18n/coleitr.cpp b/icu4c/source/i18n/coleitr.cpp
index 9d4b1ad24f1..48c1da9015a 100644
--- a/icu4c/source/i18n/coleitr.cpp
+++ b/icu4c/source/i18n/coleitr.cpp
@@ -398,8 +398,8 @@ class MaxExpSink : public ContractionsAndExpansions::CESink {
 public:
     MaxExpSink(UHashtable *h, UErrorCode &ec) : maxExpansions(h), errorCode(ec) {}
     virtual ~MaxExpSink();
-    virtual void handleCE(int64_t /*ce*/) {}
-    virtual void handleExpansion(const int64_t ces[], int32_t length) {
+    virtual void handleCE(int64_t /*ce*/) override {}
+    virtual void handleExpansion(const int64_t ces[], int32_t length) override {
         if (length <= 1) {
             // We do not need to add single CEs into the map.
             return;
diff --git a/icu4c/source/i18n/coll.cpp b/icu4c/source/i18n/coll.cpp
index 55f59ce6ecc..fe73118da03 100644
--- a/icu4c/source/i18n/coll.cpp
+++ b/icu4c/source/i18n/coll.cpp
@@ -129,7 +129,7 @@ class ICUCollatorFactory : public ICUResourceBundleFactory {
     ICUCollatorFactory() : ICUResourceBundleFactory(UnicodeString(U_ICUDATA_COLL, -1, US_INV)) { }
     virtual ~ICUCollatorFactory();
  protected:
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const;
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const override;
 };
 
 ICUCollatorFactory::~ICUCollatorFactory() {}
@@ -162,11 +162,11 @@ public:
 
     virtual ~ICUCollatorService();
 
-    virtual UObject* cloneInstance(UObject* instance) const {
+    virtual UObject* cloneInstance(UObject* instance) const override {
         return ((Collator*)instance)->clone();
     }
     
-    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* actualID, UErrorCode& status) const {
+    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* actualID, UErrorCode& status) const override {
         LocaleKey& lkey = (LocaleKey&)key;
         if (actualID) {
             // Ugly Hack Alert! We return an empty actualID to signal
@@ -179,7 +179,7 @@ public:
         return Collator::makeInstance(loc, status);
     }
     
-    virtual UObject* getKey(ICUServiceKey& key, UnicodeString* actualReturn, UErrorCode& status) const {
+    virtual UObject* getKey(ICUServiceKey& key, UnicodeString* actualReturn, UErrorCode& status) const override {
         UnicodeString ar;
         if (actualReturn == NULL) {
             actualReturn = &ar;
@@ -187,7 +187,7 @@ public:
         return (Collator*)ICULocaleService::getKey(key, actualReturn, status);
     }
 
-    virtual UBool isDefault() const {
+    virtual UBool isDefault() const override {
         return countFactories() == 1;
     }
 };
@@ -721,10 +721,10 @@ public:
 
     virtual ~CFactory();
 
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const;
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const override;
     
 protected:
-    virtual const Hashtable* getSupportedIDs(UErrorCode& status) const
+    virtual const Hashtable* getSupportedIDs(UErrorCode& status) const override
     {
         if (U_SUCCESS(status)) {
             return _ids;
@@ -733,7 +733,7 @@ protected:
     }
     
     virtual UnicodeString&
-        getDisplayName(const UnicodeString& id, const Locale& locale, UnicodeString& result) const;
+        getDisplayName(const UnicodeString& id, const Locale& locale, UnicodeString& result) const override;
 };
 
 CFactory::~CFactory()
@@ -803,7 +803,7 @@ private:
     int32_t index;
 public:
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 public:
     CollationLocaleListEnumeration()
         : index(0)
@@ -814,7 +814,7 @@ public:
 
     virtual ~CollationLocaleListEnumeration();
 
-    virtual StringEnumeration * clone() const
+    virtual StringEnumeration * clone() const override
     {
         CollationLocaleListEnumeration *result = new CollationLocaleListEnumeration();
         if (result) {
@@ -823,11 +823,11 @@ public:
         return result;
     }
 
-    virtual int32_t count(UErrorCode &/*status*/) const {
+    virtual int32_t count(UErrorCode &/*status*/) const override {
         return availableLocaleListCount;
     }
 
-    virtual const char* next(int32_t* resultLength, UErrorCode& /*status*/) {
+    virtual const char* next(int32_t* resultLength, UErrorCode& /*status*/) override {
         const char* result;
         if(index < availableLocaleListCount) {
             result = availableLocaleList[index++].getName();
@@ -843,13 +843,13 @@ public:
         return result;
     }
 
-    virtual const UnicodeString* snext(UErrorCode& status) {
+    virtual const UnicodeString* snext(UErrorCode& status) override {
         int32_t resultLength = 0;
         const char *s = next(&resultLength, status);
         return setChars(s, resultLength, status);
     }
 
-    virtual void reset(UErrorCode& /*status*/) {
+    virtual void reset(UErrorCode& /*status*/) override {
         index = 0;
     }
 };
diff --git a/icu4c/source/i18n/collationbuilder.cpp b/icu4c/source/i18n/collationbuilder.cpp
index b411b4f12f6..f5b29d1c372 100644
--- a/icu4c/source/i18n/collationbuilder.cpp
+++ b/icu4c/source/i18n/collationbuilder.cpp
@@ -59,7 +59,7 @@ public:
     virtual void getRules(
             const char *localeID, const char *collationType,
             UnicodeString &rules,
-            const char *&errorReason, UErrorCode &errorCode);
+            const char *&errorReason, UErrorCode &errorCode) override;
 };
 
 BundleImporter::~BundleImporter() {}
@@ -1581,7 +1581,7 @@ class CEFinalizer : public CollationDataBuilder::CEModifier {
 public:
     CEFinalizer(const int64_t *ces) : finalCEs(ces) {}
     virtual ~CEFinalizer();
-    virtual int64_t modifyCE32(uint32_t ce32) const {
+    virtual int64_t modifyCE32(uint32_t ce32) const override {
         U_ASSERT(!Collation::isSpecialCE32(ce32));
         if(CollationBuilder::isTempCE32(ce32)) {
             // retain case bits
@@ -1590,7 +1590,7 @@ public:
             return Collation::NO_CE;
         }
     }
-    virtual int64_t modifyCE(int64_t ce) const {
+    virtual int64_t modifyCE(int64_t ce) const override {
         if(CollationBuilder::isTempCE(ce)) {
             // retain case bits
             return finalCEs[CollationBuilder::indexFromTempCE(ce)] | (ce & 0xc000);
diff --git a/icu4c/source/i18n/collationbuilder.h b/icu4c/source/i18n/collationbuilder.h
index 82f0459748f..59d3c5d24b0 100644
--- a/icu4c/source/i18n/collationbuilder.h
+++ b/icu4c/source/i18n/collationbuilder.h
@@ -57,7 +57,7 @@ private:
 
     /** Implements CollationRuleParser::Sink. */
     virtual void addReset(int32_t strength, const UnicodeString &str,
-                          const char *&errorReason, UErrorCode &errorCode);
+                          const char *&errorReason, UErrorCode &errorCode) override;
     /**
      * Returns the secondary or tertiary weight preceding the current node's weight.
      * node=nodes[index].
@@ -70,7 +70,7 @@ private:
     /** Implements CollationRuleParser::Sink. */
     virtual void addRelation(int32_t strength, const UnicodeString &prefix,
                              const UnicodeString &str, const UnicodeString &extension,
-                             const char *&errorReason, UErrorCode &errorCode);
+                             const char *&errorReason, UErrorCode &errorCode) override;
 
     /**
      * Picks one of the current CEs and finds or inserts a node in the graph
@@ -115,11 +115,11 @@ private:
 
     /** Implements CollationRuleParser::Sink. */
     virtual void suppressContractions(const UnicodeSet &set, const char *&parserErrorReason,
-                                      UErrorCode &errorCode);
+                                      UErrorCode &errorCode) override;
 
     /** Implements CollationRuleParser::Sink. */
     virtual void optimize(const UnicodeSet &set, const char *&parserErrorReason,
-                          UErrorCode &errorCode);
+                          UErrorCode &errorCode) override;
 
     /**
      * Adds the mapping and its canonical closure.
diff --git a/icu4c/source/i18n/collationdatabuilder.cpp b/icu4c/source/i18n/collationdatabuilder.cpp
index 1d42fbc94bd..a1f3225dfdd 100644
--- a/icu4c/source/i18n/collationdatabuilder.cpp
+++ b/icu4c/source/i18n/collationdatabuilder.cpp
@@ -131,18 +131,18 @@ public:
 
     int32_t fetchCEs(const UnicodeString &str, int32_t start, int64_t ces[], int32_t cesLength);
 
-    virtual void resetToOffset(int32_t newOffset);
-    virtual int32_t getOffset() const;
+    virtual void resetToOffset(int32_t newOffset) override;
+    virtual int32_t getOffset() const override;
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
-    virtual uint32_t getDataCE32(UChar32 c) const;
-    virtual uint32_t getCE32FromBuilderData(uint32_t ce32, UErrorCode &errorCode);
+    virtual uint32_t getDataCE32(UChar32 c) const override;
+    virtual uint32_t getCE32FromBuilderData(uint32_t ce32, UErrorCode &errorCode) override;
 
     CollationDataBuilder &builder;
     CollationData builderData;
diff --git a/icu4c/source/i18n/collationkeys.h b/icu4c/source/i18n/collationkeys.h
index c526a4f14f2..5b41d14c0b6 100644
--- a/icu4c/source/i18n/collationkeys.h
+++ b/icu4c/source/i18n/collationkeys.h
@@ -38,7 +38,7 @@ public:
 
     void IgnoreBytes(int32_t numIgnore) { ignore_ = numIgnore; }
 
-    virtual void Append(const char *bytes, int32_t n);
+    virtual void Append(const char *bytes, int32_t n) override;
     void Append(uint32_t b) {
         if (ignore_ > 0) {
             --ignore_;
@@ -52,7 +52,7 @@ public:
     virtual char *GetAppendBuffer(int32_t min_capacity,
                                   int32_t desired_capacity_hint,
                                   char *scratch, int32_t scratch_capacity,
-                                  int32_t *result_capacity);
+                                  int32_t *result_capacity) override;
     int32_t NumberOfBytesAppended() const { return appended_; }
 
     /**
diff --git a/icu4c/source/i18n/coptccal.h b/icu4c/source/i18n/coptccal.h
index e9e812dbc7f..5c51af04ca0 100644
--- a/icu4c/source/i18n/coptccal.h
+++ b/icu4c/source/i18n/coptccal.h
@@ -145,14 +145,14 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @internal
      */
-    virtual CopticCalendar* clone() const;
+    virtual CopticCalendar* clone() const override;
 
     /**
      * return the calendar type, "coptic"
      * @return calendar type
      * @internal
      */
-    const char * getType() const;
+    const char * getType() const override;
 
 protected:
     //-------------------------------------------------------------------------
@@ -163,32 +163,32 @@ protected:
      * Return the extended year defined by the current fields.
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
 
     /**
      * Compute fields from the JD
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
 
     /**
      * Returns the date of the start of the default century
      * @return start of century - in milliseconds since epoch, 1970
      * @internal
      */
-    virtual UDate defaultCenturyStart() const;
+    virtual UDate defaultCenturyStart() const override;
 
     /**
      * Returns the year in which the default century begins
      * @internal
      */
-    virtual int32_t defaultCenturyStartYear() const;
+    virtual int32_t defaultCenturyStartYear() const override;
 
     /**
      * Return the date offset from Julian
      * @internal
      */
-    virtual int32_t getJDEpochOffset() const;
+    virtual int32_t getJDEpochOffset() const override;
 
 
 public:
@@ -202,7 +202,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @internal
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
diff --git a/icu4c/source/i18n/cpdtrans.h b/icu4c/source/i18n/cpdtrans.h
index 5ed74fcdd12..af60cb827e3 100644
--- a/icu4c/source/i18n/cpdtrans.h
+++ b/icu4c/source/i18n/cpdtrans.h
@@ -98,7 +98,7 @@ public:
     /**
      * Transliterator API.
      */
-    virtual CompoundTransliterator* clone() const;
+    virtual CompoundTransliterator* clone() const override;
 
     /**
      * Returns the number of transliterators in this chain.
@@ -137,33 +137,33 @@ public:
      * U+000A, U+0020..U+007E.
      */
     virtual UnicodeString& toRules(UnicodeString& result,
-                                   UBool escapeUnprintable) const;
+                                   UBool escapeUnprintable) const override;
 
  protected:
     /**
      * Implement Transliterator framework
      */
-    virtual void handleGetSourceSet(UnicodeSet& result) const;
+    virtual void handleGetSourceSet(UnicodeSet& result) const override;
 
  public:
     /**
      * Override Transliterator framework
      */
-    virtual UnicodeSet& getTargetSet(UnicodeSet& result) const;
+    virtual UnicodeSet& getTargetSet(UnicodeSet& result) const override;
 
 protected:
     /**
      * Implements {@link Transliterator#handleTransliterate}.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& idx,
-                                     UBool incremental) const;
+                                     UBool incremental) const override;
 
 public:
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/csr2022.h b/icu4c/source/i18n/csr2022.h
index aa803ce9a4d..4418728f0ec 100644
--- a/icu4c/source/i18n/csr2022.h
+++ b/icu4c/source/i18n/csr2022.h
@@ -62,9 +62,9 @@ class CharsetRecog_2022JP :public CharsetRecog_2022
 public:
     virtual ~CharsetRecog_2022JP();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    UBool match(InputText *textIn, CharsetMatch *results) const;
+    UBool match(InputText *textIn, CharsetMatch *results) const override;
 };
 
 #if !UCONFIG_ONLY_HTML_CONVERSION
@@ -72,9 +72,9 @@ class CharsetRecog_2022KR :public CharsetRecog_2022 {
 public:
     virtual ~CharsetRecog_2022KR();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    UBool match(InputText *textIn, CharsetMatch *results) const;
+    UBool match(InputText *textIn, CharsetMatch *results) const override;
 
 };
 
@@ -83,9 +83,9 @@ class CharsetRecog_2022CN :public CharsetRecog_2022
 public:
     virtual ~CharsetRecog_2022CN();
 
-    const char* getName() const;
+    const char* getName() const override;
 
-    UBool match(InputText *textIn, CharsetMatch *results) const;
+    UBool match(InputText *textIn, CharsetMatch *results) const override;
 };
 #endif
 
diff --git a/icu4c/source/i18n/csrsbcs.h b/icu4c/source/i18n/csrsbcs.h
index 18908efe71e..96f982c59bd 100644
--- a/icu4c/source/i18n/csrsbcs.h
+++ b/icu4c/source/i18n/csrsbcs.h
@@ -63,8 +63,8 @@ public:
 private:
     int32_t alef;
     int32_t isLamAlef(int32_t b);
-    int32_t nextByte(InputText *det);
-    void parseCharacters(InputText *det);
+    int32_t nextByte(InputText *det) override;
+    void parseCharacters(InputText *det) override;
 };
 #endif
 
@@ -83,23 +83,23 @@ class CharsetRecog_8859_1 : public CharsetRecog_sbcs
 {
 public:
     virtual ~CharsetRecog_8859_1();
-    const char *getName() const;
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    const char *getName() const override;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_2 : public CharsetRecog_sbcs
 {
 public:
     virtual ~CharsetRecog_8859_2();
-    const char *getName() const;
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    const char *getName() const override;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_5 : public CharsetRecog_sbcs
 {
 public:
     virtual ~CharsetRecog_8859_5();
-    const char *getName() const;
+    const char *getName() const override;
 };
 
 class CharsetRecog_8859_6 : public CharsetRecog_sbcs
@@ -107,7 +107,7 @@ class CharsetRecog_8859_6 : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_8859_6();
 
-    const char *getName() const;
+    const char *getName() const override;
 };
 
 class CharsetRecog_8859_7 : public CharsetRecog_sbcs
@@ -115,7 +115,7 @@ class CharsetRecog_8859_7 : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_8859_7();
 
-    const char *getName() const;
+    const char *getName() const override;
 };
 
 class CharsetRecog_8859_8 : public CharsetRecog_sbcs
@@ -123,7 +123,7 @@ class CharsetRecog_8859_8 : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_8859_8();
 	
-    virtual const char *getName() const;
+    virtual const char *getName() const override;
 };
 
 class CharsetRecog_8859_9 : public CharsetRecog_sbcs
@@ -131,7 +131,7 @@ class CharsetRecog_8859_9 : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_8859_9();
 
-    const char *getName() const;
+    const char *getName() const override;
 };
 
 
@@ -141,9 +141,9 @@ class CharsetRecog_8859_5_ru : public CharsetRecog_8859_5
 public:
     virtual ~CharsetRecog_8859_5_ru();
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_6_ar : public CharsetRecog_8859_6
@@ -151,9 +151,9 @@ class CharsetRecog_8859_6_ar : public CharsetRecog_8859_6
 public:
     virtual ~CharsetRecog_8859_6_ar();
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_7_el : public CharsetRecog_8859_7
@@ -161,9 +161,9 @@ class CharsetRecog_8859_7_el : public CharsetRecog_8859_7
 public:
     virtual ~CharsetRecog_8859_7_el();
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_8_I_he : public CharsetRecog_8859_8
@@ -171,11 +171,11 @@ class CharsetRecog_8859_8_I_he : public CharsetRecog_8859_8
 public:
     virtual ~CharsetRecog_8859_8_I_he();
 	
-    const char *getName() const;
+    const char *getName() const override;
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_8_he : public CharsetRecog_8859_8
@@ -183,9 +183,9 @@ class CharsetRecog_8859_8_he : public CharsetRecog_8859_8
 public:
     virtual ~CharsetRecog_8859_8_he ();
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_8859_9_tr : public CharsetRecog_8859_9
@@ -193,9 +193,9 @@ class CharsetRecog_8859_9_tr : public CharsetRecog_8859_9
 public:
     virtual ~CharsetRecog_8859_9_tr ();
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_windows_1256 : public CharsetRecog_sbcs
@@ -203,11 +203,11 @@ class CharsetRecog_windows_1256 : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_windows_1256();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_windows_1251 : public CharsetRecog_sbcs
@@ -215,11 +215,11 @@ class CharsetRecog_windows_1251 : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_windows_1251();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 
@@ -228,11 +228,11 @@ class CharsetRecog_KOI8_R : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_KOI8_R();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 #if !UCONFIG_ONLY_HTML_CONVERSION
@@ -241,24 +241,24 @@ class CharsetRecog_IBM424_he : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_IBM424_he();
 
-    const char *getLanguage() const;
+    const char *getLanguage() const override;
 };
 
 class CharsetRecog_IBM424_he_rtl : public CharsetRecog_IBM424_he {
 public:
     virtual ~CharsetRecog_IBM424_he_rtl();
     
-    const char *getName() const;
+    const char *getName() const override;
     
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_IBM424_he_ltr : public CharsetRecog_IBM424_he {
     virtual ~CharsetRecog_IBM424_he_ltr();
     
-    const char *getName() const;
+    const char *getName() const override;
     
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_IBM420_ar : public CharsetRecog_sbcs
@@ -266,8 +266,8 @@ class CharsetRecog_IBM420_ar : public CharsetRecog_sbcs
 public:
     virtual ~CharsetRecog_IBM420_ar();
 
-    const char *getLanguage() const;
-	int32_t match_sbcs(InputText *det, const int32_t ngrams[], const uint8_t charMap[]) const;
+    const char *getLanguage() const override;
+	int32_t match_sbcs(InputText *det, const int32_t ngrams[], const uint8_t charMap[]) const override;
     
 };
 
@@ -275,17 +275,17 @@ class CharsetRecog_IBM420_ar_rtl : public CharsetRecog_IBM420_ar {
 public:
     virtual ~CharsetRecog_IBM420_ar_rtl();
     
-    const char *getName() const;
+    const char *getName() const override;
     
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_IBM420_ar_ltr : public CharsetRecog_IBM420_ar {
     virtual ~CharsetRecog_IBM420_ar_ltr();
     
-    const char *getName() const;
+    const char *getName() const override;
     
-    virtual UBool match(InputText *det, CharsetMatch *results) const;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override;
 };
 #endif
 
diff --git a/icu4c/source/i18n/csrutf8.h b/icu4c/source/i18n/csrutf8.h
index 6089eb6f75c..bcfb38ac951 100644
--- a/icu4c/source/i18n/csrutf8.h
+++ b/icu4c/source/i18n/csrutf8.h
@@ -29,12 +29,12 @@ class CharsetRecog_UTF8: public CharsetRecognizer {
 		
     virtual ~CharsetRecog_UTF8();		 
 
-    const char *getName() const;
+    const char *getName() const override;
 
     /* (non-Javadoc)
      * @see com.ibm.icu.text.CharsetRecognizer#match(com.ibm.icu.text.CharsetDetector)
      */
-    UBool match(InputText *input, CharsetMatch *results) const;
+    UBool match(InputText *input, CharsetMatch *results) const override;
 	
 };
 
diff --git a/icu4c/source/i18n/currfmt.h b/icu4c/source/i18n/currfmt.h
index 69a031957bc..2a75cae2bb3 100644
--- a/icu4c/source/i18n/currfmt.h
+++ b/icu4c/source/i18n/currfmt.h
@@ -57,7 +57,7 @@ class CurrencyFormat : public MeasureFormat {
     /**
      * Override Format API.
      */
-    virtual CurrencyFormat* clone() const;
+    virtual CurrencyFormat* clone() const override;
 
 
     using MeasureFormat::format;
@@ -68,19 +68,19 @@ class CurrencyFormat : public MeasureFormat {
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPosition& pos,
-                                  UErrorCode& ec) const;
+                                  UErrorCode& ec) const override;
 
     /**
      * Override Format API.
      */
     virtual void parseObject(const UnicodeString& source,
                              Formattable& result,
-                             ParsePosition& pos) const;
+                             ParsePosition& pos) const override;
 
     /**
      * Override Format API.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * Returns the class ID for this class.
diff --git a/icu4c/source/i18n/dangical.h b/icu4c/source/i18n/dangical.h
index ece805e36d3..4d55a5b59ee 100644
--- a/icu4c/source/i18n/dangical.h
+++ b/icu4c/source/i18n/dangical.h
@@ -66,7 +66,7 @@ class DangiCalendar : public ChineseCalendar {
    * Clone.
    * @internal
    */
-  virtual DangiCalendar* clone() const;
+  virtual DangiCalendar* clone() const override;
 
   //----------------------------------------------------------------------
   // Internal methods & astronomical calculations
@@ -83,7 +83,7 @@ class DangiCalendar : public ChineseCalendar {
    *           same class ID. Objects of other classes have different class IDs.
    * @internal
    */
-  virtual UClassID getDynamicClassID(void) const;
+  virtual UClassID getDynamicClassID(void) const override;
 
   /**
    * Return the class ID for this class. This is useful only for comparing to a return
@@ -104,7 +104,7 @@ class DangiCalendar : public ChineseCalendar {
    * @return calendar type
    * @internal
    */
-  const char * getType() const;
+  const char * getType() const override;
 
 
  private:
diff --git a/icu4c/source/i18n/datefmt.cpp b/icu4c/source/i18n/datefmt.cpp
index 46d5f110a6b..fed8f79aa0f 100644
--- a/icu4c/source/i18n/datefmt.cpp
+++ b/icu4c/source/i18n/datefmt.cpp
@@ -69,7 +69,7 @@ class U_I18N_API DateFmtBestPatternKey : public LocaleCacheKey<DateFmtBestPatter
 private:
     UnicodeString fSkeleton;
 protected:
-    virtual bool equals(const CacheKeyBase &other) const {
+    virtual bool equals(const CacheKeyBase &other) const override {
        if (!LocaleCacheKey<DateFmtBestPattern>::equals(other)) {
            return false;
        }
@@ -87,17 +87,17 @@ public:
             LocaleCacheKey<DateFmtBestPattern>(other),
             fSkeleton(other.fSkeleton) { }
     virtual ~DateFmtBestPatternKey();
-    virtual int32_t hashCode() const {
+    virtual int32_t hashCode() const override {
         return (int32_t)(37u * (uint32_t)LocaleCacheKey<DateFmtBestPattern>::hashCode() + (uint32_t)fSkeleton.hashCode());
     }
     inline bool operator==(const DateFmtBestPatternKey &other) const {
         return fSkeleton == other.fSkeleton;
     }
-    virtual CacheKeyBase *clone() const {
+    virtual CacheKeyBase *clone() const override {
         return new DateFmtBestPatternKey(*this);
     }
     virtual const DateFmtBestPattern *createObject(
-            const void * /*unused*/, UErrorCode &status) const {
+            const void * /*unused*/, UErrorCode &status) const override {
         LocalPointer<DateTimePatternGenerator> dtpg(
                     DateTimePatternGenerator::createInstance(fLoc, status));
         if (U_FAILURE(status)) {
diff --git a/icu4c/source/i18n/dayperiodrules.cpp b/icu4c/source/i18n/dayperiodrules.cpp
index e364ecb708c..efe92dd794b 100644
--- a/icu4c/source/i18n/dayperiodrules.cpp
+++ b/icu4c/source/i18n/dayperiodrules.cpp
@@ -50,7 +50,7 @@ struct DayPeriodRulesDataSink : public ResourceSink {
     }
     virtual ~DayPeriodRulesDataSink();
 
-    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) {
+    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) override {
         ResourceTable dayPeriodData = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
 
@@ -282,7 +282,7 @@ struct DayPeriodRulesDataSink : public ResourceSink {
 struct DayPeriodRulesCountSink : public ResourceSink {
     virtual ~DayPeriodRulesCountSink();
 
-    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) {
+    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) override {
         ResourceTable rules = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
 
diff --git a/icu4c/source/i18n/dcfmtsym.cpp b/icu4c/source/i18n/dcfmtsym.cpp
index 96be4388efe..4f5bae4e11e 100644
--- a/icu4c/source/i18n/dcfmtsym.cpp
+++ b/icu4c/source/i18n/dcfmtsym.cpp
@@ -232,7 +232,7 @@ struct DecFmtSymDataSink : public ResourceSink {
     virtual ~DecFmtSymDataSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         ResourceTable symbolsTable = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
         for (int32_t j = 0; symbolsTable.getKeyAndValue(j, key, value); ++j) {
@@ -287,7 +287,7 @@ struct CurrencySpacingSink : public ResourceSink {
     virtual ~CurrencySpacingSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         ResourceTable spacingTypesTable = value.getTable(errorCode);
         for (int32_t i = 0; spacingTypesTable.getKeyAndValue(i, key, value); ++i) {
             UBool beforeCurrency;
diff --git a/icu4c/source/i18n/dtfmtsym.cpp b/icu4c/source/i18n/dtfmtsym.cpp
index 501c9a19535..ab5f2b612c1 100644
--- a/icu4c/source/i18n/dtfmtsym.cpp
+++ b/icu4c/source/i18n/dtfmtsym.cpp
@@ -1545,7 +1545,7 @@ struct CalendarDataSink : public ResourceSink {
         aliasPathPairs.removeAllElements();
     }
 
-    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) {
+    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) override {
         if (U_FAILURE(errorCode)) { return; }
         U_ASSERT(!currentCalendarType.isEmpty());
 
diff --git a/icu4c/source/i18n/dtitvinf.cpp b/icu4c/source/i18n/dtitvinf.cpp
index 3a85658095c..cd3ef6a57b0 100644
--- a/icu4c/source/i18n/dtitvinf.cpp
+++ b/icu4c/source/i18n/dtitvinf.cpp
@@ -240,7 +240,7 @@ struct DateIntervalInfo::DateIntervalSink : public ResourceSink {
             : dateIntervalInfo(diInfo), nextCalendarType(currentCalendarType, -1, US_INV) { }
     virtual ~DateIntervalSink();
 
-    virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/, UErrorCode &errorCode) {
+    virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/, UErrorCode &errorCode) override {
         if (U_FAILURE(errorCode)) { return; }
 
         // Iterate over all the calendar entries and only pick the 'intervalFormats' table.
diff --git a/icu4c/source/i18n/dtptngen.cpp b/icu4c/source/i18n/dtptngen.cpp
index a74add021cf..2acbca16eda 100644
--- a/icu4c/source/i18n/dtptngen.cpp
+++ b/icu4c/source/i18n/dtptngen.cpp
@@ -525,7 +525,7 @@ struct AllowedHourFormatsSink : public ResourceSink {
     virtual ~AllowedHourFormatsSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-                     UErrorCode &errorCode) {
+                     UErrorCode &errorCode) override {
         ResourceTable timeData = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
         for (int32_t i = 0; timeData.getKeyAndValue(i, key, value); ++i) {
@@ -933,7 +933,7 @@ struct DateTimePatternGenerator::AppendItemFormatsSink : public ResourceSink {
     virtual ~AppendItemFormatsSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         UDateTimePatternField field = dtpg.getAppendFormatNumber(key);
         if (field == UDATPG_FIELD_COUNT) { return; }
         const UnicodeString& valueStr = value.getUnicodeString(errorCode);
@@ -962,7 +962,7 @@ struct DateTimePatternGenerator::AppendItemNamesSink : public ResourceSink {
     virtual ~AppendItemNamesSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         UDateTimePGDisplayWidth width;
         UDateTimePatternField field = dtpg.getFieldAndWidthIndices(key, &width);
         if (field == UDATPG_FIELD_COUNT) { return; }
@@ -1014,7 +1014,7 @@ struct DateTimePatternGenerator::AvailableFormatsSink : public ResourceSink {
     virtual ~AvailableFormatsSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool isRoot,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         const UnicodeString formatKey(key, -1, US_INV);
         if (!dtpg.isAvailableFormatSet(formatKey) ) {
             dtpg.setAvailableFormat(formatKey, errorCode);
diff --git a/icu4c/source/i18n/dtptngen_impl.h b/icu4c/source/i18n/dtptngen_impl.h
index 7692689c15b..5caae11654b 100644
--- a/icu4c/source/i18n/dtptngen_impl.h
+++ b/icu4c/source/i18n/dtptngen_impl.h
@@ -279,10 +279,10 @@ public:
     DTSkeletonEnumeration(PatternMap& patternMap, dtStrEnum type, UErrorCode& status);
     virtual ~DTSkeletonEnumeration();
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
-    virtual const UnicodeString* snext(UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual UClassID getDynamicClassID(void) const override;
+    virtual const UnicodeString* snext(UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
 private:
     int32_t pos;
     UBool isCanonicalItem(const UnicodeString& item);
@@ -294,10 +294,10 @@ public:
     DTRedundantEnumeration();
     virtual ~DTRedundantEnumeration();
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
-    virtual const UnicodeString* snext(UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual UClassID getDynamicClassID(void) const override;
+    virtual const UnicodeString* snext(UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
     void add(const UnicodeString &pattern, UErrorCode& status);
 private:
     int32_t pos;
diff --git a/icu4c/source/i18n/esctrn.h b/icu4c/source/i18n/esctrn.h
index 2a2c6dcfe9a..a4282ea86a2 100644
--- a/icu4c/source/i18n/esctrn.h
+++ b/icu4c/source/i18n/esctrn.h
@@ -115,12 +115,12 @@ class EscapeTransliterator : public Transliterator {
     /**
      * Transliterator API.
      */
-    virtual EscapeTransliterator* clone() const;
+    virtual EscapeTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -133,7 +133,7 @@ class EscapeTransliterator : public Transliterator {
      * Implements {@link Transliterator#handleTransliterate}.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                             UBool isIncremental) const;
+                             UBool isIncremental) const override;
 
 };
 
diff --git a/icu4c/source/i18n/ethpccal.h b/icu4c/source/i18n/ethpccal.h
index d1e8f424b57..0cc5b6c535e 100644
--- a/icu4c/source/i18n/ethpccal.h
+++ b/icu4c/source/i18n/ethpccal.h
@@ -141,14 +141,14 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @internal
      */
-    virtual EthiopicCalendar* clone() const;
+    virtual EthiopicCalendar* clone() const override;
 
     /**
      * return the calendar type, "ethiopic"
      * @return calendar type
      * @internal
      */
-    virtual const char * getType() const;
+    virtual const char * getType() const override;
 
     /**
      * Set Alem or Mihret era.
@@ -173,38 +173,38 @@ protected:
      * Return the extended year defined by the current fields.
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
 
     /**
      * Compute fields from the JD
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
 
     /**
      * Calculate the limit for a specified type of limit and field
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
 
     /**
      * Returns the date of the start of the default century
      * @return start of century - in milliseconds since epoch, 1970
      * @internal
      */
-    virtual UDate defaultCenturyStart() const;
+    virtual UDate defaultCenturyStart() const override;
 
     /**
      * Returns the year in which the default century begins
      * @internal
      */
-    virtual int32_t defaultCenturyStartYear() const;
+    virtual int32_t defaultCenturyStartYear() const override;
 
     /**
      * Return the date offset from Julian
      * @internal
      */
-    virtual int32_t getJDEpochOffset() const;
+    virtual int32_t getJDEpochOffset() const override;
 
 private:
     /**
@@ -229,7 +229,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @internal
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
diff --git a/icu4c/source/i18n/funcrepl.h b/icu4c/source/i18n/funcrepl.h
index fe41f6caaa6..529a10ebbf5 100644
--- a/icu4c/source/i18n/funcrepl.h
+++ b/icu4c/source/i18n/funcrepl.h
@@ -70,13 +70,13 @@ class FunctionReplacer : public UnicodeFunctor, public UnicodeReplacer {
     /**
      * Implement UnicodeFunctor
      */
-    virtual FunctionReplacer* clone() const;
+    virtual FunctionReplacer* clone() const override;
 
     /**
      * UnicodeFunctor API.  Cast 'this' to a UnicodeReplacer* pointer
      * and return the pointer.
      */
-    virtual UnicodeReplacer* toReplacer() const;
+    virtual UnicodeReplacer* toReplacer() const override;
 
     /**
      * UnicodeReplacer API
@@ -84,28 +84,28 @@ class FunctionReplacer : public UnicodeFunctor, public UnicodeReplacer {
     virtual int32_t replace(Replaceable& text,
                             int32_t start,
                             int32_t limit,
-                            int32_t& cursor);
+                            int32_t& cursor) override;
 
     /**
      * UnicodeReplacer API
      */
     virtual UnicodeString& toReplacerPattern(UnicodeString& rule,
-                                             UBool escapeUnprintable) const;
+                                             UBool escapeUnprintable) const override;
 
     /**
      * Implement UnicodeReplacer
      */
-    virtual void addReplacementSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addReplacementSetTo(UnicodeSet& toUnionTo) const override;
 
     /**
      * UnicodeFunctor API
      */
-    virtual void setData(const TransliterationRuleData*);
+    virtual void setData(const TransliterationRuleData*) override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/hebrwcal.h b/icu4c/source/i18n/hebrwcal.h
index d27e6b879d6..ae4401832d4 100644
--- a/icu4c/source/i18n/hebrwcal.h
+++ b/icu4c/source/i18n/hebrwcal.h
@@ -192,7 +192,7 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @internal
      */
-    virtual HebrewCalendar* clone() const;
+    virtual HebrewCalendar* clone() const override;
     
 public:
     /**
@@ -205,7 +205,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @internal
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
@@ -226,7 +226,7 @@ public:
      * @return calendar type
      * @internal
      */
-    virtual const char * getType() const;
+    virtual const char * getType() const override;
 
 
     // Calendar API
@@ -242,11 +242,11 @@ public:
      *                previously set in the time field is invalid, this will be set to
      *                an error status.
      */
-    virtual void add(UCalendarDateFields field, int32_t amount, UErrorCode& status);
+    virtual void add(UCalendarDateFields field, int32_t amount, UErrorCode& status) override;
     /**
      * @deprecated ICU 2.6 use UCalendarDateFields instead of EDateFields
      */
-    virtual void add(EDateFields field, int32_t amount, UErrorCode& status);
+    virtual void add(EDateFields field, int32_t amount, UErrorCode& status) override;
 
 
     /**
@@ -260,7 +260,7 @@ public:
      *                an error status.
      * @internal
      */
-    virtual void roll(UCalendarDateFields field, int32_t amount, UErrorCode& status);
+    virtual void roll(UCalendarDateFields field, int32_t amount, UErrorCode& status) override;
 
     /**
      * (Overrides Calendar) Rolls up or down by the given amount in the specified field.
@@ -273,7 +273,7 @@ public:
      *                an error status.
      * @deprecated ICU 2.6. Use roll(UCalendarDateFields field, int32_t amount, UErrorCode& status) instead.
 `     */
-    virtual void roll(EDateFields field, int32_t amount, UErrorCode& status);
+    virtual void roll(EDateFields field, int32_t amount, UErrorCode& status) override;
 
     /**
      * @internal 
@@ -303,7 +303,7 @@ public:
      * <code>LEAST_MAXIMUM</code>, or <code>MAXIMUM</code>
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
 
     /**
      * Return the number of days in the given month of the given extended
@@ -312,7 +312,7 @@ public:
      * implementation than the default implementation in Calendar.
      * @internal
      */
-    virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const;
+    virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const override;
 
     /**
      * Return the number of days in the given extended year of this
@@ -321,7 +321,7 @@ public:
      * default implementation in Calendar.
      * @stable ICU 2.0
      */
-    virtual int32_t handleGetYearLength(int32_t eyear) const;
+    virtual int32_t handleGetYearLength(int32_t eyear) const override;
     /**
      * Subclasses may override this method to compute several fields
      * specific to each calendar system.  These are:
@@ -337,7 +337,7 @@ public:
      * a calendar with the specified Julian/Gregorian cutover date.
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
     /**
      * Return the extended year defined by the current fields.  This will
      * use the UCAL_EXTENDED_YEAR field or the UCAL_YEAR and supra-year fields (such
@@ -346,7 +346,7 @@ public:
      * @return the extended year
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
     /**
      * Return the Julian day number of day before the first day of the
      * given month in the given extended year.  Subclasses should override
@@ -361,7 +361,7 @@ public:
      * @internal
      */
     virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month,
-                                                   UBool useMonth) const;
+                                                   UBool useMonth) const override;
 
 
     /**
@@ -370,7 +370,7 @@ public:
      * special handling for month validation for Hebrew calendar.
      * @internal
      */
-    virtual void validateField(UCalendarDateFields field, UErrorCode &status);
+    virtual void validateField(UCalendarDateFields field, UErrorCode &status) override;
 
  protected:
 
@@ -383,26 +383,26 @@ public:
    *           false, otherwise.
    * @internal
    */
-  virtual UBool inDaylightTime(UErrorCode& status) const;
+  virtual UBool inDaylightTime(UErrorCode& status) const override;
 
-    /**
-     * Returns true because the Hebrew Calendar does have a default century
-     * @internal
-     */
-    virtual UBool haveDefaultCentury() const;
+  /**
+   * Returns true because the Hebrew Calendar does have a default century
+   * @internal
+   */
+  virtual UBool haveDefaultCentury() const override;
 
-    /**
-     * Returns the date of the start of the default century
-     * @return start of century - in milliseconds since epoch, 1970
-     * @internal
-     */
-    virtual UDate defaultCenturyStart() const;
+  /**
+   * Returns the date of the start of the default century
+   * @return start of century - in milliseconds since epoch, 1970
+   * @internal
+   */
+  virtual UDate defaultCenturyStart() const override;
 
-    /**
-     * Returns the year in which the default century begins
-     * @internal
-     */
-    virtual int32_t defaultCenturyStartYear() const;
+  /**
+   * Returns the year in which the default century begins
+   * @internal
+   */
+  virtual int32_t defaultCenturyStartYear() const override;
 
  private: // Calendar-specific implementation
     /**
diff --git a/icu4c/source/i18n/indiancal.h b/icu4c/source/i18n/indiancal.h
index 1ada0a47028..029515cf547 100644
--- a/icu4c/source/i18n/indiancal.h
+++ b/icu4c/source/i18n/indiancal.h
@@ -186,7 +186,7 @@ public:
   // TODO: copy c'tor, etc
 
   // clone
-  virtual IndianCalendar* clone() const;
+  virtual IndianCalendar* clone() const override;
 
  private:
   /**
@@ -200,7 +200,7 @@ public:
   /**
    * @internal
    */
-  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
   
   /**
    * Return the length (in days) of the given month.
@@ -209,13 +209,13 @@ public:
    * @param year  The month(0-based) in Indian year
    * @internal
    */
-  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const;
+  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const override;
   
   /**
    * Return the number of days in the given Indian year
    * @internal
    */
-  virtual int32_t handleGetYearLength(int32_t extendedYear) const;
+  virtual int32_t handleGetYearLength(int32_t extendedYear) const override;
 
   //-------------------------------------------------------------------------
   // Functions for converting from field values to milliseconds....
@@ -225,7 +225,7 @@ public:
   /**
    * @internal
    */
-  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const;
+  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const override;
 
   //-------------------------------------------------------------------------
   // Functions for converting from milliseconds to field values
@@ -234,7 +234,7 @@ public:
   /**
    * @internal
    */
-  virtual int32_t handleGetExtendedYear();
+  virtual int32_t handleGetExtendedYear() override;
 
   /**
    * Override Calendar to compute several fields specific to the Indian
@@ -252,7 +252,7 @@ public:
    * calendar equivalents for the given Julian day.
    * @internal
    */
-  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
 
   // UObject stuff
  public: 
@@ -261,7 +261,7 @@ public:
    *           same class ID. Objects of other classes have different class IDs.
    * @internal
    */
-  virtual UClassID getDynamicClassID(void) const;
+  virtual UClassID getDynamicClassID(void) const override;
 
   /**
    * Return the class ID for this class. This is useful only for comparing to a return
@@ -282,7 +282,7 @@ public:
    * @return calendar type
    * @internal
    */
-  virtual const char * getType() const;
+  virtual const char * getType() const override;
 
 private:
   IndianCalendar(); // default constructor not implemented
@@ -299,27 +299,27 @@ protected:
    *           false, otherwise.
    * @internal
    */
-  virtual UBool inDaylightTime(UErrorCode& status) const;
+  virtual UBool inDaylightTime(UErrorCode& status) const override;
 
 
   /**
    * Returns true because the Indian Calendar does have a default century
    * @internal
    */
-  virtual UBool haveDefaultCentury() const;
+  virtual UBool haveDefaultCentury() const override;
 
   /**
    * Returns the date of the start of the default century
    * @return start of century - in milliseconds since epoch, 1970
    * @internal
    */
-  virtual UDate defaultCenturyStart() const;
+  virtual UDate defaultCenturyStart() const override;
 
   /**
    * Returns the year in which the default century begins
    * @internal
    */
-  virtual int32_t defaultCenturyStartYear() const;
+  virtual int32_t defaultCenturyStartYear() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/islamcal.h b/icu4c/source/i18n/islamcal.h
index 7a360e1076b..a29b7daa086 100644
--- a/icu4c/source/i18n/islamcal.h
+++ b/icu4c/source/i18n/islamcal.h
@@ -230,7 +230,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
   // TODO: copy c'tor, etc
 
   // clone
-  virtual IslamicCalendar* clone() const;
+  virtual IslamicCalendar* clone() const override;
 
  private:
   /**
@@ -292,7 +292,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
   /**
    * @internal
    */
-  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
   
   /**
    * Return the length (in days) of the given month.
@@ -301,13 +301,13 @@ class U_I18N_API IslamicCalendar : public Calendar {
    * @param year  The hijri month, 0-based
    * @internal
    */
-  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const;
+  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const override;
   
   /**
    * Return the number of days in the given Islamic year
    * @internal
    */
-  virtual int32_t handleGetYearLength(int32_t extendedYear) const;
+  virtual int32_t handleGetYearLength(int32_t extendedYear) const override;
     
   //-------------------------------------------------------------------------
   // Functions for converting from field values to milliseconds....
@@ -317,7 +317,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
   /**
    * @internal
    */
-  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const;
+  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const override;
 
   //-------------------------------------------------------------------------
   // Functions for converting from milliseconds to field values
@@ -326,7 +326,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
   /**
    * @internal
    */
-  virtual int32_t handleGetExtendedYear();
+  virtual int32_t handleGetExtendedYear() override;
 
   /**
    * Override Calendar to compute several fields specific to the Islamic
@@ -344,7 +344,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
    * calendar equivalents for the given Julian day.
    * @internal
    */
-  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
 
   // UObject stuff
  public: 
@@ -353,7 +353,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
    *           same class ID. Objects of other classes have different class IDs.
    * @internal
    */
-  virtual UClassID getDynamicClassID(void) const;
+  virtual UClassID getDynamicClassID(void) const override;
 
   /**
    * Return the class ID for this class. This is useful only for comparing to a return
@@ -374,7 +374,7 @@ class U_I18N_API IslamicCalendar : public Calendar {
    * @return calendar type
    * @internal
    */
-  virtual const char * getType() const;
+  virtual const char * getType() const override;
 
  private:
   IslamicCalendar(); // default constructor not implemented
@@ -391,27 +391,27 @@ class U_I18N_API IslamicCalendar : public Calendar {
    *           false, otherwise.
    * @internal
    */
-  virtual UBool inDaylightTime(UErrorCode& status) const;
+  virtual UBool inDaylightTime(UErrorCode& status) const override;
 
 
   /**
    * Returns true because the Islamic Calendar does have a default century
    * @internal
    */
-  virtual UBool haveDefaultCentury() const;
+  virtual UBool haveDefaultCentury() const override;
 
   /**
    * Returns the date of the start of the default century
    * @return start of century - in milliseconds since epoch, 1970
    * @internal
    */
-  virtual UDate defaultCenturyStart() const;
+  virtual UDate defaultCenturyStart() const override;
 
   /**
    * Returns the year in which the default century begins
    * @internal
    */
-  virtual int32_t defaultCenturyStartYear() const;
+  virtual int32_t defaultCenturyStartYear() const override;
 
  private:
   /**
diff --git a/icu4c/source/i18n/japancal.h b/icu4c/source/i18n/japancal.h
index e5b95d732c0..88513440528 100644
--- a/icu4c/source/i18n/japancal.h
+++ b/icu4c/source/i18n/japancal.h
@@ -116,20 +116,20 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @internal
      */
-    virtual JapaneseCalendar* clone() const;
+    virtual JapaneseCalendar* clone() const override;
 
     /**
      * Return the extended year defined by the current fields.  In the 
      * Japanese calendar case, this is equal to the equivalent extended Gregorian year.
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
 
     /**
      * Return the maximum value that this field could have, given the current date.
      * @internal
      */
-    virtual int32_t getActualMaximum(UCalendarDateFields field, UErrorCode& status) const;
+    virtual int32_t getActualMaximum(UCalendarDateFields field, UErrorCode& status) const override;
 
 
 public:
@@ -143,7 +143,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @internal
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
@@ -164,24 +164,24 @@ public:
      * @return calendar type
      * @internal
      */
-    virtual const char * getType() const;
+    virtual const char * getType() const override;
 
     /**
      * @return false - no default century in Japanese
      * @internal 
      */
-    virtual UBool haveDefaultCentury() const;
+    virtual UBool haveDefaultCentury() const override;
 
     /**
      * Not used - no default century.
      * @internal
      */
-    virtual UDate defaultCenturyStart() const;
+    virtual UDate defaultCenturyStart() const override;
     /**
      * Not used - no default century.
      * @internal
      */
-    virtual int32_t defaultCenturyStartYear() const;
+    virtual int32_t defaultCenturyStartYear() const override;
 
 private:
     JapaneseCalendar(); // default constructor not implemented
@@ -191,19 +191,19 @@ protected:
      * Calculate the era for internal computation
      * @internal
      */
-    virtual int32_t internalGetEra() const;
+    virtual int32_t internalGetEra() const override;
 
     /**
      * Compute fields from the JD
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode& status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode& status) override;
 
     /**
      * Calculate the limit for a specified type of limit and field
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
 
     /***
      * Called by computeJulianDay.  Returns the default month (0-based) for the year,
@@ -212,7 +212,7 @@ protected:
      * @param eyear the extended year
      * @internal
      */
-    virtual int32_t getDefaultMonthInYear(int32_t eyear);
+    virtual int32_t getDefaultMonthInYear(int32_t eyear) override;
 
     /***
      * Called by computeJulianDay.  Returns the default day (1-based) for the month,
@@ -222,7 +222,7 @@ protected:
      * @param mon the month in the year
      * @internal
      */
-    virtual int32_t getDefaultDayInMonth(int32_t eyear, int32_t month);
+    virtual int32_t getDefaultDayInMonth(int32_t eyear, int32_t month) override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/listformatter.cpp b/icu4c/source/i18n/listformatter.cpp
index e5c01c0ab32..4142fa461da 100644
--- a/icu4c/source/i18n/listformatter.cpp
+++ b/icu4c/source/i18n/listformatter.cpp
@@ -444,7 +444,7 @@ struct ListFormatter::ListPatternsSink : public ResourceSink {
     }
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         aliasedStyle[0] = 0;
         if (value.getType() == URES_ALIAS) {
             setAliasedStyle(value.getAliasUnicodeString(errorCode));
diff --git a/icu4c/source/i18n/measunit_extra.cpp b/icu4c/source/i18n/measunit_extra.cpp
index 6c82ffbc8e0..e543c745681 100644
--- a/icu4c/source/i18n/measunit_extra.cpp
+++ b/icu4c/source/i18n/measunit_extra.cpp
@@ -187,7 +187,7 @@ class SimpleUnitIdentifiersSink : public icu::ResourceSink {
      * @param noFallback Ignored.
      * @param status The standard ICU error code output parameter.
      */
-    void put(const char * /*key*/, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) {
+    void put(const char * /*key*/, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) override {
         ResourceTable table = value.getTable(status);
         if (U_FAILURE(status)) return;
 
@@ -275,7 +275,7 @@ class CategoriesSink : public icu::ResourceSink {
     explicit CategoriesSink(const UChar **out, int32_t &outSize, BytesTrieBuilder &trieBuilder)
         : outQuantitiesArray(out), outSize(outSize), trieBuilder(trieBuilder), outIndex(0) {}
 
-    void put(const char * /*key*/, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) {
+    void put(const char * /*key*/, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) override {
         ResourceArray array = value.getArray(status);
         if (U_FAILURE(status)) {
             return;
diff --git a/icu4c/source/i18n/msgfmt_impl.h b/icu4c/source/i18n/msgfmt_impl.h
index 1cece1a0943..57988389132 100644
--- a/icu4c/source/i18n/msgfmt_impl.h
+++ b/icu4c/source/i18n/msgfmt_impl.h
@@ -29,10 +29,10 @@ public:
     FormatNameEnumeration(UVector *fFormatNames, UErrorCode& status);
     virtual ~FormatNameEnumeration();
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
-    virtual const UnicodeString* snext(UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual UClassID getDynamicClassID(void) const override;
+    virtual const UnicodeString* snext(UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
 private:
     int32_t pos;
     UVector *fFormatNames;
diff --git a/icu4c/source/i18n/name2uni.h b/icu4c/source/i18n/name2uni.h
index 44ad85fb822..6881c6bc857 100644
--- a/icu4c/source/i18n/name2uni.h
+++ b/icu4c/source/i18n/name2uni.h
@@ -49,12 +49,12 @@ public:
      * Transliterator API.
      * @return    A copy of the object.
      */
-    virtual NameUnicodeTransliterator* clone() const;
+    virtual NameUnicodeTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -73,7 +73,7 @@ public:
      *                      pos.contextLimit. Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
     /**
      * Set of characters which occur in Unicode character names.
diff --git a/icu4c/source/i18n/nfsubs.cpp b/icu4c/source/i18n/nfsubs.cpp
index a40a27694d7..70ac18be468 100644
--- a/icu4c/source/i18n/nfsubs.cpp
+++ b/icu4c/source/i18n/nfsubs.cpp
@@ -58,15 +58,15 @@ public:
         UErrorCode& status);
     virtual ~SameValueSubstitution();
 
-    virtual int64_t transformNumber(int64_t number) const { return number; }
-    virtual double transformNumber(double number) const { return number; }
-    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const { return newRuleValue; }
-    virtual double calcUpperBound(double oldUpperBound) const { return oldUpperBound; }
-    virtual UChar tokenChar() const { return (UChar)0x003d; } // '='
+    virtual int64_t transformNumber(int64_t number) const override { return number; }
+    virtual double transformNumber(double number) const override { return number; }
+    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const override { return newRuleValue; }
+    virtual double calcUpperBound(double oldUpperBound) const override { return oldUpperBound; }
+    virtual UChar tokenChar() const override { return (UChar)0x003d; } // '='
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 SameValueSubstitution::~SameValueSubstitution() {}
@@ -88,7 +88,7 @@ public:
     }
     virtual ~MultiplierSubstitution();
 
-    virtual void setDivisor(int32_t radix, int16_t exponent, UErrorCode& status) { 
+    virtual void setDivisor(int32_t radix, int16_t exponent, UErrorCode& status) override {
         divisor = util64_pow(radix, exponent);
 
         if(divisor == 0) {
@@ -96,13 +96,13 @@ public:
         }
     }
 
-    virtual bool operator==(const NFSubstitution& rhs) const;
+    virtual bool operator==(const NFSubstitution& rhs) const override;
 
-    virtual int64_t transformNumber(int64_t number) const {
+    virtual int64_t transformNumber(int64_t number) const override {
         return number / divisor;
     }
 
-    virtual double transformNumber(double number) const {
+    virtual double transformNumber(double number) const override {
         if (getRuleSet()) {
             return uprv_floor(number / divisor);
         } else {
@@ -110,17 +110,17 @@ public:
         }
     }
 
-    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const {
+    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const override {
         return newRuleValue * divisor;
     }
 
-    virtual double calcUpperBound(double /*oldUpperBound*/) const { return static_cast<double>(divisor); }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const override { return static_cast<double>(divisor); }
 
-    virtual UChar tokenChar() const { return (UChar)0x003c; } // '<'
+    virtual UChar tokenChar() const override { return (UChar)0x003c; } // '<'
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 MultiplierSubstitution::~MultiplierSubstitution() {}
@@ -137,7 +137,7 @@ public:
         UErrorCode& status);
     virtual ~ModulusSubstitution();
 
-    virtual void setDivisor(int32_t radix, int16_t exponent, UErrorCode& status) { 
+    virtual void setDivisor(int32_t radix, int16_t exponent, UErrorCode& status) override {
         divisor = util64_pow(radix, exponent);
 
         if (divisor == 0) {
@@ -145,13 +145,13 @@ public:
         }
     }
 
-    virtual bool operator==(const NFSubstitution& rhs) const;
+    virtual bool operator==(const NFSubstitution& rhs) const override;
 
-    virtual void doSubstitution(int64_t number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const;
-    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const;
+    virtual void doSubstitution(int64_t number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const override;
+    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const override;
 
-    virtual int64_t transformNumber(int64_t number) const { return number % divisor; }
-    virtual double transformNumber(double number) const { return uprv_fmod(number, static_cast<double>(divisor)); }
+    virtual int64_t transformNumber(int64_t number) const override { return number % divisor; }
+    virtual double transformNumber(double number) const override { return uprv_fmod(number, static_cast<double>(divisor)); }
 
     virtual UBool doParse(const UnicodeString& text, 
         ParsePosition& parsePosition,
@@ -159,23 +159,23 @@ public:
         double upperBound,
         UBool lenientParse,
         uint32_t nonNumericalExecutedRuleMask,
-        Formattable& result) const;
+        Formattable& result) const override;
 
-    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const {
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const override {
         return oldRuleValue - uprv_fmod(oldRuleValue, static_cast<double>(divisor)) + newRuleValue;
     }
 
-    virtual double calcUpperBound(double /*oldUpperBound*/) const { return static_cast<double>(divisor); }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const override { return static_cast<double>(divisor); }
 
-    virtual UBool isModulusSubstitution() const { return TRUE; }
+    virtual UBool isModulusSubstitution() const override { return TRUE; }
 
-    virtual UChar tokenChar() const { return (UChar)0x003e; } // '>'
+    virtual UChar tokenChar() const override { return (UChar)0x003e; } // '>'
 
-	virtual void toString(UnicodeString& result) const;
+    virtual void toString(UnicodeString& result) const override;
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 ModulusSubstitution::~ModulusSubstitution() {}
@@ -189,15 +189,15 @@ public:
         : NFSubstitution(_pos, _ruleSet, description, status) {}
     virtual ~IntegralPartSubstitution();
 
-    virtual int64_t transformNumber(int64_t number) const { return number; }
-    virtual double transformNumber(double number) const { return uprv_floor(number); }
-    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue + oldRuleValue; }
-    virtual double calcUpperBound(double /*oldUpperBound*/) const { return DBL_MAX; }
-    virtual UChar tokenChar() const { return (UChar)0x003c; } // '<'
+    virtual int64_t transformNumber(int64_t number) const override { return number; }
+    virtual double transformNumber(double number) const override { return uprv_floor(number); }
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const override { return newRuleValue + oldRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const override { return DBL_MAX; }
+    virtual UChar tokenChar() const override { return (UChar)0x003c; } // '<'
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 IntegralPartSubstitution::~IntegralPartSubstitution() {}
@@ -213,12 +213,12 @@ public:
         UErrorCode& status);
     virtual ~FractionalPartSubstitution();
 
-    virtual bool operator==(const NFSubstitution& rhs) const;
+    virtual bool operator==(const NFSubstitution& rhs) const override;
 
-    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const;
-    virtual void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/, int32_t /*recursionCount*/, UErrorCode& /*status*/) const {}
-    virtual int64_t transformNumber(int64_t /*number*/) const { return 0; }
-    virtual double transformNumber(double number) const { return number - uprv_floor(number); }
+    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const override;
+    virtual void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/, int32_t /*recursionCount*/, UErrorCode& /*status*/) const override {}
+    virtual int64_t transformNumber(int64_t /*number*/) const override { return 0; }
+    virtual double transformNumber(double number) const override { return number - uprv_floor(number); }
 
     virtual UBool doParse(const UnicodeString& text,
         ParsePosition& parsePosition,
@@ -226,15 +226,15 @@ public:
         double upperBound,
         UBool lenientParse,
         uint32_t nonNumericalExecutedRuleMask,
-        Formattable& result) const;
+        Formattable& result) const override;
 
-    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue + oldRuleValue; }
-    virtual double calcUpperBound(double /*oldUpperBound*/) const { return 0.0; }
-    virtual UChar tokenChar() const { return (UChar)0x003e; } // '>'
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const override { return newRuleValue + oldRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const override { return 0.0; }
+    virtual UChar tokenChar() const override { return (UChar)0x003e; } // '>'
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 FractionalPartSubstitution::~FractionalPartSubstitution() {}
@@ -248,15 +248,15 @@ public:
         : NFSubstitution(_pos, _ruleSet, description, status) {}
     virtual ~AbsoluteValueSubstitution();
 
-    virtual int64_t transformNumber(int64_t number) const { return number >= 0 ? number : -number; }
-    virtual double transformNumber(double number) const { return uprv_fabs(number); }
-    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const { return -newRuleValue; }
-    virtual double calcUpperBound(double /*oldUpperBound*/) const { return DBL_MAX; }
-    virtual UChar tokenChar() const { return (UChar)0x003e; } // '>'
+    virtual int64_t transformNumber(int64_t number) const override { return number >= 0 ? number : -number; }
+    virtual double transformNumber(double number) const override { return uprv_fabs(number); }
+    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const override { return -newRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const override { return DBL_MAX; }
+    virtual UChar tokenChar() const override { return (UChar)0x003e; } // '>'
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 AbsoluteValueSubstitution::~AbsoluteValueSubstitution() {}
@@ -285,30 +285,30 @@ public:
     }
     virtual ~NumeratorSubstitution();
 
-    virtual bool operator==(const NFSubstitution& rhs) const;
+    virtual bool operator==(const NFSubstitution& rhs) const override;
 
-    virtual int64_t transformNumber(int64_t number) const { return number * ldenominator; }
-    virtual double transformNumber(double number) const { return uprv_round(number * denominator); }
+    virtual int64_t transformNumber(int64_t number) const override { return number * ldenominator; }
+    virtual double transformNumber(double number) const override { return uprv_round(number * denominator); }
 
-    virtual void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/, int32_t /*recursionCount*/, UErrorCode& /*status*/) const {}
-    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const;
+    virtual void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/, int32_t /*recursionCount*/, UErrorCode& /*status*/) const override {}
+    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos, int32_t recursionCount, UErrorCode& status) const override;
     virtual UBool doParse(const UnicodeString& text, 
         ParsePosition& parsePosition,
         double baseValue,
         double upperBound,
         UBool /*lenientParse*/,
         uint32_t nonNumericalExecutedRuleMask,
-        Formattable& result) const;
+        Formattable& result) const override;
 
-    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue / oldRuleValue; }
-    virtual double calcUpperBound(double /*oldUpperBound*/) const { return denominator; }
-    virtual UChar tokenChar() const { return (UChar)0x003c; } // '<'
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const override { return newRuleValue / oldRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const override { return denominator; }
+    virtual UChar tokenChar() const override { return (UChar)0x003c; } // '<'
 private:
     static const UChar LTLT[2];
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 NumeratorSubstitution::~NumeratorSubstitution() {}
diff --git a/icu4c/source/i18n/nfsubs.h b/icu4c/source/i18n/nfsubs.h
index 49eb0a07042..0abd98be097 100644
--- a/icu4c/source/i18n/nfsubs.h
+++ b/icu4c/source/i18n/nfsubs.h
@@ -250,7 +250,7 @@ private:
 
 public:
     static UClassID getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/nortrans.h b/icu4c/source/i18n/nortrans.h
index 198ed29c95e..01cb97ab38b 100644
--- a/icu4c/source/i18n/nortrans.h
+++ b/icu4c/source/i18n/nortrans.h
@@ -44,12 +44,12 @@ class NormalizationTransliterator : public Transliterator {
      * Transliterator API.
      * @return    A copy of the object.
      */
-    virtual NormalizationTransliterator* clone() const;
+    virtual NormalizationTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -68,7 +68,7 @@ class NormalizationTransliterator : public Transliterator {
      *                      pos.contextLimit. Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                             UBool isIncremental) const;
+                             UBool isIncremental) const override;
  public:
 
     /**
diff --git a/icu4c/source/i18n/nultrans.h b/icu4c/source/i18n/nultrans.h
index 36c92fa7b18..f5f2fbc9117 100644
--- a/icu4c/source/i18n/nultrans.h
+++ b/icu4c/source/i18n/nultrans.h
@@ -45,19 +45,19 @@ public:
      * Transliterator API.
      * @internal Use transliterator factory methods instead since this class will be removed in that release.
      */
-    virtual NullTransliterator* clone() const;
+    virtual NullTransliterator* clone() const override;
 
     /**
      * Implements {@link Transliterator#handleTransliterate}.
      * @internal Use transliterator factory methods instead since this class will be removed in that release.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/numfmt.cpp b/icu4c/source/i18n/numfmt.cpp
index bbc9c78b408..b8ce1c930f9 100644
--- a/icu4c/source/i18n/numfmt.cpp
+++ b/icu4c/source/i18n/numfmt.cpp
@@ -860,7 +860,7 @@ class ICUNumberFormatFactory : public ICUResourceBundleFactory {
 public:
     virtual ~ICUNumberFormatFactory();
 protected:
-    virtual UObject* handleCreate(const Locale& loc, int32_t kind, const ICUService* /* service */, UErrorCode& status) const {
+    virtual UObject* handleCreate(const Locale& loc, int32_t kind, const ICUService* /* service */, UErrorCode& status) const override {
         return NumberFormat::makeInstance(loc, (UNumberFormatStyle)kind, status);
     }
 };
@@ -884,7 +884,7 @@ public:
 
     virtual ~NFFactory();
 
-    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const
+    virtual UObject* create(const ICUServiceKey& key, const ICUService* service, UErrorCode& status) const override
     {
         if (handlesKey(key, status)) {
             const LocaleKey& lkey = (const LocaleKey&)key;
@@ -907,7 +907,7 @@ protected:
      * otherwise).  This can be called often and might need to be
      * cached if it is expensive to create.
      */
-    virtual const Hashtable* getSupportedIDs(UErrorCode& status) const
+    virtual const Hashtable* getSupportedIDs(UErrorCode& status) const override
     {
         if (U_SUCCESS(status)) {
             if (!_ids) {
@@ -943,11 +943,11 @@ public:
 
     virtual ~ICUNumberFormatService();
 
-    virtual UObject* cloneInstance(UObject* instance) const {
+    virtual UObject* cloneInstance(UObject* instance) const override {
         return ((NumberFormat*)instance)->clone();
     }
 
-    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* /* actualID */, UErrorCode& status) const {
+    virtual UObject* handleDefault(const ICUServiceKey& key, UnicodeString* /* actualID */, UErrorCode& status) const override {
         LocaleKey& lkey = (LocaleKey&)key;
         int32_t kind = lkey.kind();
         Locale loc;
@@ -955,7 +955,7 @@ public:
         return NumberFormat::makeInstance(loc, (UNumberFormatStyle)kind, status);
     }
 
-    virtual UBool isDefault() const {
+    virtual UBool isDefault() const override {
         return countFactories() == 1;
     }
 };
diff --git a/icu4c/source/i18n/olsontz.h b/icu4c/source/i18n/olsontz.h
index 207fa8144f7..75d86781edd 100644
--- a/icu4c/source/i18n/olsontz.h
+++ b/icu4c/source/i18n/olsontz.h
@@ -146,12 +146,12 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
     /**
      * Returns true if the two TimeZone objects are equal.
      */
-    virtual bool operator==(const TimeZone& other) const;
+    virtual bool operator==(const TimeZone& other) const override;
 
     /**
      * TimeZone API.
      */
-    virtual OlsonTimeZone* clone() const;
+    virtual OlsonTimeZone* clone() const override;
 
     /**
      * TimeZone API.
@@ -161,14 +161,14 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
     /**
      * TimeZone API.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
     
     /**
      * TimeZone API.  Do not call this; prefer getOffset(UDate,...).
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month,
                               int32_t day, uint8_t dayOfWeek,
-                              int32_t millis, UErrorCode& ec) const;
+                              int32_t millis, UErrorCode& ec) const override;
 
     /**
      * TimeZone API.  Do not call this; prefer getOffset(UDate,...).
@@ -176,13 +176,13 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month,
                               int32_t day, uint8_t dayOfWeek,
                               int32_t millis, int32_t monthLength,
-                              UErrorCode& ec) const;
+                              UErrorCode& ec) const override;
 
     /**
      * TimeZone API.
      */
     virtual void getOffset(UDate date, UBool local, int32_t& rawOffset,
-                   int32_t& dstOffset, UErrorCode& ec) const;
+                   int32_t& dstOffset, UErrorCode& ec) const override;
 
     /**
      * BasicTimeZone API.
@@ -190,14 +190,14 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
     virtual void getOffsetFromLocal(
         UDate date, UTimeZoneLocalOption nonExistingTimeOpt,
         UTimeZoneLocalOption duplicatedTimeOpt,
-        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const;
+        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const override;
 
     /**
      * TimeZone API.  This method has no effect since objects of this
      * class are quasi-immutable (the base class allows the ID to be
      * changed).
      */
-    virtual void setRawOffset(int32_t offsetMillis);
+    virtual void setRawOffset(int32_t offsetMillis) override;
 
     /**
      * TimeZone API.  For a historical zone, the raw offset can change
@@ -205,7 +205,7 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
      * expected behavior, this method returns the raw offset for the
      * current moment in time.
      */
-    virtual int32_t getRawOffset() const;
+    virtual int32_t getRawOffset() const override;
 
     /**
      * TimeZone API.  For a historical zone, whether DST is used or
@@ -213,22 +213,22 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
      * behavior, this method returns true if DST is observed at any
      * point in the current year.
      */
-    virtual UBool useDaylightTime() const;
+    virtual UBool useDaylightTime() const override;
 
     /**
      * TimeZone API.
      */
-    virtual UBool inDaylightTime(UDate date, UErrorCode& ec) const;
+    virtual UBool inDaylightTime(UDate date, UErrorCode& ec) const override;
 
     /**
      * TimeZone API.
      */
-    virtual int32_t getDSTSavings() const;
+    virtual int32_t getDSTSavings() const override;
 
     /**
      * TimeZone API.  Also comare historic transitions.
      */
-    virtual UBool hasSameRules(const TimeZone& other) const;
+    virtual UBool hasSameRules(const TimeZone& other) const override;
 
     /**
      * BasicTimeZone API.
@@ -238,7 +238,7 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
      * @param result    Receives the first transition after the base time.
      * @return  true if the transition is found.
      */
-    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * BasicTimeZone API.
@@ -248,7 +248,7 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
      * @param result    Receives the most recent transition before the base time.
      * @return  true if the transition is found.
      */
-    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * BasicTimeZone API.
@@ -258,7 +258,7 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
      * @param status    Receives error status code.
      * @return The number of <code>TimeZoneRule</code>s representing time transitions.
      */
-    virtual int32_t countTransitionRules(UErrorCode& status) const;
+    virtual int32_t countTransitionRules(UErrorCode& status) const override;
 
     /**
      * Gets the <code>InitialTimeZoneRule</code> and the set of <code>TimeZoneRule</code>
@@ -276,7 +276,7 @@ class U_I18N_API OlsonTimeZone: public BasicTimeZone {
      * @param status        Receives error status code.
      */
     virtual void getTimeZoneRules(const InitialTimeZoneRule*& initial,
-        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const;
+        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const override;
 
     /**
      * Internal API returning the canonical ID of this zone.
diff --git a/icu4c/source/i18n/persncal.h b/icu4c/source/i18n/persncal.h
index c9eff1c3058..d0f2ee5ec20 100644
--- a/icu4c/source/i18n/persncal.h
+++ b/icu4c/source/i18n/persncal.h
@@ -164,7 +164,7 @@ class PersianCalendar : public Calendar {
   // TODO: copy c'tor, etc
 
   // clone
-  virtual PersianCalendar* clone() const;
+  virtual PersianCalendar* clone() const override;
 
  private:
   /**
@@ -194,7 +194,7 @@ class PersianCalendar : public Calendar {
   /**
    * @internal
    */
-  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+  virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
   
   /**
    * Return the length (in days) of the given month.
@@ -203,13 +203,13 @@ class PersianCalendar : public Calendar {
    * @param year  The hijri shamsi month, 0-based
    * @internal
    */
-  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const;
+  virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const override;
   
   /**
    * Return the number of days in the given Persian year
    * @internal
    */
-  virtual int32_t handleGetYearLength(int32_t extendedYear) const;
+  virtual int32_t handleGetYearLength(int32_t extendedYear) const override;
     
   //-------------------------------------------------------------------------
   // Functions for converting from field values to milliseconds....
@@ -219,7 +219,7 @@ class PersianCalendar : public Calendar {
   /**
    * @internal
    */
-  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const;
+  virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month, UBool useMonth) const override;
 
   //-------------------------------------------------------------------------
   // Functions for converting from milliseconds to field values
@@ -228,7 +228,7 @@ class PersianCalendar : public Calendar {
   /**
    * @internal
    */
-  virtual int32_t handleGetExtendedYear();
+  virtual int32_t handleGetExtendedYear() override;
 
   /**
    * Override Calendar to compute several fields specific to the Persian
@@ -246,7 +246,7 @@ class PersianCalendar : public Calendar {
    * calendar equivalents for the given Julian day.
    * @internal
    */
-  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+  virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
 
   // UObject stuff
  public: 
@@ -255,7 +255,7 @@ class PersianCalendar : public Calendar {
    *           same class ID. Objects of other classes have different class IDs.
    * @internal
    */
-  virtual UClassID getDynamicClassID(void) const;
+  virtual UClassID getDynamicClassID(void) const override;
 
   /**
    * Return the class ID for this class. This is useful only for comparing to a return
@@ -276,7 +276,7 @@ class PersianCalendar : public Calendar {
    * @return calendar type
    * @internal
    */
-  virtual const char * getType() const;
+  virtual const char * getType() const override;
 
  private:
   PersianCalendar(); // default constructor not implemented
@@ -292,26 +292,26 @@ class PersianCalendar : public Calendar {
    *           false, otherwise.
    * @internal
    */
-  virtual UBool inDaylightTime(UErrorCode& status) const;
+  virtual UBool inDaylightTime(UErrorCode& status) const override;
 
   /**
    * Returns true because the Persian Calendar does have a default century
    * @internal
    */
-  virtual UBool haveDefaultCentury() const;
+  virtual UBool haveDefaultCentury() const override;
 
   /**
    * Returns the date of the start of the default century
    * @return start of century - in milliseconds since epoch, 1970
    * @internal
    */
-  virtual UDate defaultCenturyStart() const;
+  virtual UDate defaultCenturyStart() const override;
 
   /**
    * Returns the year in which the default century begins
    * @internal
    */
-  virtual int32_t defaultCenturyStartYear() const;
+  virtual int32_t defaultCenturyStartYear() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/plurrule_impl.h b/icu4c/source/i18n/plurrule_impl.h
index 5668e42be9a..1a30096f2f4 100644
--- a/icu4c/source/i18n/plurrule_impl.h
+++ b/icu4c/source/i18n/plurrule_impl.h
@@ -408,10 +408,10 @@ public:
     PluralKeywordEnumeration(RuleChain *header, UErrorCode& status);
     virtual ~PluralKeywordEnumeration();
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
-    virtual const UnicodeString* snext(UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual UClassID getDynamicClassID(void) const override;
+    virtual const UnicodeString* snext(UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
 private:
     int32_t         pos;
     UVector         fKeywordNames;
@@ -422,9 +422,9 @@ class U_I18N_API PluralAvailableLocalesEnumeration: public StringEnumeration {
   public:
     PluralAvailableLocalesEnumeration(UErrorCode &status);
     virtual ~PluralAvailableLocalesEnumeration();
-    virtual const char* next(int32_t *resultLength, UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual const char* next(int32_t *resultLength, UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
   private:
     UErrorCode      fOpenStatus;
     UResourceBundle *fLocales = nullptr;
diff --git a/icu4c/source/i18n/quant.h b/icu4c/source/i18n/quant.h
index df6924cc127..427a6b04808 100644
--- a/icu4c/source/i18n/quant.h
+++ b/icu4c/source/i18n/quant.h
@@ -39,13 +39,13 @@ class Quantifier : public UnicodeFunctor, public UnicodeMatcher {
      * and return the pointer.
      * @return the UnicodeMatcher pointer.
      */
-    virtual UnicodeMatcher* toMatcher() const;
+    virtual UnicodeMatcher* toMatcher() const override;
 
     /**
      * Implement UnicodeFunctor
      * @return a copy of the object.
      */
-    virtual Quantifier* clone() const;
+    virtual Quantifier* clone() const override;
 
     /**
      * Implement UnicodeMatcher
@@ -72,7 +72,7 @@ class Quantifier : public UnicodeFunctor, public UnicodeMatcher {
     virtual UMatchDegree matches(const Replaceable& text,
                                  int32_t& offset,
                                  int32_t limit,
-                                 UBool incremental);
+                                 UBool incremental) override;
 
     /**
      * Implement UnicodeMatcher
@@ -81,29 +81,29 @@ class Quantifier : public UnicodeFunctor, public UnicodeMatcher {
      * @return                  A reference to 'result'.
      */
     virtual UnicodeString& toPattern(UnicodeString& result,
-                                     UBool escapeUnprintable = false) const;
+                                     UBool escapeUnprintable = false) const override;
 
     /**
      * Implement UnicodeMatcher
      * @param v    the given index value.
      * @return     true if this rule matches the given index value.
      */
-    virtual UBool matchesIndexValue(uint8_t v) const;
+    virtual UBool matchesIndexValue(uint8_t v) const override;
 
     /**
      * Implement UnicodeMatcher
      */
-    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const override;
 
     /**
      * UnicodeFunctor API
      */
-    virtual void setData(const TransliterationRuleData*);
+    virtual void setData(const TransliterationRuleData*) override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/rbnf.cpp b/icu4c/source/i18n/rbnf.cpp
index 4d5ca403f20..7f54fd7a33f 100644
--- a/icu4c/source/i18n/rbnf.cpp
+++ b/icu4c/source/i18n/rbnf.cpp
@@ -271,11 +271,11 @@ public:
     static StringLocalizationInfo* create(const UnicodeString& info, UParseError& perror, UErrorCode& status);
     
     virtual ~StringLocalizationInfo();
-    virtual int32_t getNumberOfRuleSets(void) const { return numRuleSets; }
-    virtual const UChar* getRuleSetName(int32_t index) const;
-    virtual int32_t getNumberOfDisplayLocales(void) const { return numLocales; }
-    virtual const UChar* getLocaleName(int32_t index) const;
-    virtual const UChar* getDisplayName(int32_t localeIndex, int32_t ruleIndex) const;
+    virtual int32_t getNumberOfRuleSets(void) const override { return numRuleSets; }
+    virtual const UChar* getRuleSetName(int32_t index) const override;
+    virtual int32_t getNumberOfDisplayLocales(void) const override { return numLocales; }
+    virtual const UChar* getLocaleName(int32_t index) const override;
+    virtual const UChar* getDisplayName(int32_t localeIndex, int32_t ruleIndex) const override;
     
 //    virtual UClassID getDynamicClassID() const;
 //    static UClassID getStaticClassID(void);
diff --git a/icu4c/source/i18n/rbt.h b/icu4c/source/i18n/rbt.h
index 671b41e1a39..8a43c90d462 100644
--- a/icu4c/source/i18n/rbt.h
+++ b/icu4c/source/i18n/rbt.h
@@ -144,7 +144,7 @@ public:
      * Implement Transliterator API.
      * @internal Use transliterator factory methods instead since this class will be removed in that release.
      */
-    virtual RuleBasedTransliterator* clone() const;
+    virtual RuleBasedTransliterator* clone() const override;
 
 protected:
     /**
@@ -152,7 +152,7 @@ protected:
      * @internal Use transliterator factory methods instead since this class will be removed in that release.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offsets,
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
 public:
     /**
@@ -168,19 +168,19 @@ public:
      * @internal Use transliterator factory methods instead since this class will be removed in that release.
      */
     virtual UnicodeString& toRules(UnicodeString& result,
-                                   UBool escapeUnprintable) const;
+                                   UBool escapeUnprintable) const override;
 
 protected:
     /**
      * Implement Transliterator framework
      */
-    virtual void handleGetSourceSet(UnicodeSet& result) const;
+    virtual void handleGetSourceSet(UnicodeSet& result) const override;
 
 public:
     /**
      * Override Transliterator framework
      */
-    virtual UnicodeSet& getTargetSet(UnicodeSet& result) const;
+    virtual UnicodeSet& getTargetSet(UnicodeSet& result) const override;
 
     /**
      * Return the class ID for this class.  This is useful only for
@@ -205,7 +205,7 @@ public:
      * class have the same class ID.  Objects of other classes have
      * different class IDs.
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
 private:
 
diff --git a/icu4c/source/i18n/rbt_pars.cpp b/icu4c/source/i18n/rbt_pars.cpp
index 44e96c659a0..2f207a8deb0 100644
--- a/icu4c/source/i18n/rbt_pars.cpp
+++ b/icu4c/source/i18n/rbt_pars.cpp
@@ -148,12 +148,12 @@ public:
 
     virtual ~ParseData();
 
-    virtual const UnicodeString* lookup(const UnicodeString& s) const;
+    virtual const UnicodeString* lookup(const UnicodeString& s) const override;
 
-    virtual const UnicodeFunctor* lookupMatcher(UChar32 ch) const;
+    virtual const UnicodeFunctor* lookupMatcher(UChar32 ch) const override;
 
     virtual UnicodeString parseReference(const UnicodeString& text,
-                                         ParsePosition& pos, int32_t limit) const;
+                                         ParsePosition& pos, int32_t limit) const override;
     /**
      * Return true if the given character is a matcher standin or a plain
      * character (non standin).
diff --git a/icu4c/source/i18n/region_impl.h b/icu4c/source/i18n/region_impl.h
index 5e5a64529ad..62acaa4511b 100644
--- a/icu4c/source/i18n/region_impl.h
+++ b/icu4c/source/i18n/region_impl.h
@@ -29,10 +29,10 @@ public:
     RegionNameEnumeration(UVector *fNameList, UErrorCode& status);
     virtual ~RegionNameEnumeration();
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
-    virtual const UnicodeString* snext(UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual UClassID getDynamicClassID(void) const override;
+    virtual const UnicodeString* snext(UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
 private:
     int32_t pos;
     UVector *fRegionNames;
diff --git a/icu4c/source/i18n/reldatefmt.cpp b/icu4c/source/i18n/reldatefmt.cpp
index 19fabbea669..fc40d41a4a9 100644
--- a/icu4c/source/i18n/reldatefmt.cpp
+++ b/icu4c/source/i18n/reldatefmt.cpp
@@ -588,7 +588,7 @@ struct RelDateTimeFmtDataSink : public ResourceSink {
     }
 
     virtual void put(const char *key, ResourceValue &value,
-                     UBool /*noFallback*/, UErrorCode &errorCode) {
+                     UBool /*noFallback*/, UErrorCode &errorCode) override {
         // Main entry point to sink
         ResourceTable table = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
diff --git a/icu4c/source/i18n/reldtfmt.cpp b/icu4c/source/i18n/reldtfmt.cpp
index c8d68b68654..5fdef1c0d67 100644
--- a/icu4c/source/i18n/reldtfmt.cpp
+++ b/icu4c/source/i18n/reldtfmt.cpp
@@ -485,7 +485,7 @@ struct RelDateFmtDataSink : public ResourceSink {
   virtual ~RelDateFmtDataSink();
 
   virtual void put(const char *key, ResourceValue &value,
-                   UBool /*noFallback*/, UErrorCode &errorCode) {
+                   UBool /*noFallback*/, UErrorCode &errorCode) override {
       ResourceTable relDayTable = value.getTable(errorCode);
       int32_t n = 0;
       int32_t len = 0;
diff --git a/icu4c/source/i18n/reldtfmt.h b/icu4c/source/i18n/reldtfmt.h
index 72464de6d56..98b333a02be 100644
--- a/icu4c/source/i18n/reldtfmt.h
+++ b/icu4c/source/i18n/reldtfmt.h
@@ -71,7 +71,7 @@ public:
      * @return    A copy of the object.
      * @internal ICU 3.8
      */
-    virtual RelativeDateFormat* clone() const;
+    virtual RelativeDateFormat* clone() const override;
 
     /**
      * Return true if the given Format objects are semantically equal. Objects
@@ -80,7 +80,7 @@ public:
      * @return         true if the given Format objects are semantically equal.
      * @internal ICU 3.8
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
 
     using DateFormat::format;
@@ -103,7 +103,7 @@ public:
      */
     virtual UnicodeString& format(  Calendar& cal,
                                     UnicodeString& appendTo,
-                                    FieldPosition& pos) const;
+                                    FieldPosition& pos) const override;
 
     /**
      * Format an object to produce a string. This method handles Formattable
@@ -122,7 +122,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPosition& pos,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
 
 
     /**
@@ -146,7 +146,7 @@ public:
      */
     virtual void parse( const UnicodeString& text,
                         Calendar& cal,
-                        ParsePosition& pos) const;
+                        ParsePosition& pos) const override;
 
     /**
      * Parse a date/time string starting at the given parse position. For
@@ -187,7 +187,7 @@ public:
      * @internal ICU 3.8
      */
     virtual UDate parse( const UnicodeString& text,
-                        UErrorCode& status) const;
+                        UErrorCode& status) const override;
 
     /**
      * Return a single pattern string generated by combining the patterns for the
@@ -245,7 +245,7 @@ public:
      *               updated with any new status from the function. 
      * @internal ICU 53
      */
-    virtual void setContext(UDisplayContext value, UErrorCode& status);
+    virtual void setContext(UDisplayContext value, UErrorCode& status) override;
 
 private:
     SimpleDateFormat *fDateTimeFormatter;
@@ -327,7 +327,7 @@ public:
      *                  other classes have different class IDs.
      * @internal ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 
diff --git a/icu4c/source/i18n/remtrans.h b/icu4c/source/i18n/remtrans.h
index 13de01594a3..398cc5177cd 100644
--- a/icu4c/source/i18n/remtrans.h
+++ b/icu4c/source/i18n/remtrans.h
@@ -47,7 +47,7 @@ public:
      * Transliterator API.
      * @return A copy of the object.
      */
-    virtual RemoveTransliterator* clone() const;
+    virtual RemoveTransliterator* clone() const override;
 
     /**
      * Implements {@link Transliterator#handleTransliterate}.
@@ -59,12 +59,12 @@ public:
      *                      pos.contextLimit. Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/rulebasedcollator.cpp b/icu4c/source/i18n/rulebasedcollator.cpp
index d898cd52b32..5e5cc3db626 100644
--- a/icu4c/source/i18n/rulebasedcollator.cpp
+++ b/icu4c/source/i18n/rulebasedcollator.cpp
@@ -66,8 +66,8 @@ public:
     virtual ~FixedSortKeyByteSink();
 
 private:
-    virtual void AppendBeyondCapacity(const char *bytes, int32_t n, int32_t length);
-    virtual UBool Resize(int32_t appendCapacity, int32_t length);
+    virtual void AppendBeyondCapacity(const char *bytes, int32_t n, int32_t length) override;
+    virtual UBool Resize(int32_t appendCapacity, int32_t length) override;
 };
 
 FixedSortKeyByteSink::~FixedSortKeyByteSink() {}
@@ -98,8 +98,8 @@ public:
     virtual ~CollationKeyByteSink();
 
 private:
-    virtual void AppendBeyondCapacity(const char *bytes, int32_t n, int32_t length);
-    virtual UBool Resize(int32_t appendCapacity, int32_t length);
+    virtual void AppendBeyondCapacity(const char *bytes, int32_t n, int32_t length) override;
+    virtual UBool Resize(int32_t appendCapacity, int32_t length) override;
 
     CollationKey &key_;
 };
@@ -830,7 +830,7 @@ class UTF16NFDIterator : public NFDIterator {
 public:
     UTF16NFDIterator(const UChar *text, const UChar *textLimit) : s(text), limit(textLimit) {}
 protected:
-    virtual UChar32 nextRawCodePoint() {
+    virtual UChar32 nextRawCodePoint() override {
         if(s == limit) { return U_SENTINEL; }
         UChar32 c = *s++;
         if(limit == NULL && c == 0) {
@@ -882,7 +882,7 @@ public:
     UTF8NFDIterator(const uint8_t *text, int32_t textLength)
         : s(text), pos(0), length(textLength) {}
 protected:
-    virtual UChar32 nextRawCodePoint() {
+    virtual UChar32 nextRawCodePoint() override {
         if(pos == length || (s[pos] == 0 && length < 0)) { return U_SENTINEL; }
         UChar32 c;
         U8_NEXT_OR_FFFD(s, pos, length, c);
@@ -899,7 +899,7 @@ public:
     FCDUTF8NFDIterator(const CollationData *data, const uint8_t *text, int32_t textLength)
             : u8ci(data, FALSE, text, 0, textLength) {}
 protected:
-    virtual UChar32 nextRawCodePoint() {
+    virtual UChar32 nextRawCodePoint() override {
         UErrorCode errorCode = U_ZERO_ERROR;
         return u8ci.nextCodePoint(errorCode);
     }
@@ -911,7 +911,7 @@ class UIterNFDIterator : public NFDIterator {
 public:
     UIterNFDIterator(UCharIterator &it) : iter(it) {}
 protected:
-    virtual UChar32 nextRawCodePoint() {
+    virtual UChar32 nextRawCodePoint() override {
         return uiter_next32(&iter);
     }
 private:
@@ -923,7 +923,7 @@ public:
     FCDUIterNFDIterator(const CollationData *data, UCharIterator &it, int32_t startIndex)
             : uici(data, FALSE, it, startIndex) {}
 protected:
-    virtual UChar32 nextRawCodePoint() {
+    virtual UChar32 nextRawCodePoint() override {
         UErrorCode errorCode = U_ZERO_ERROR;
         return uici.nextCodePoint(errorCode);
     }
@@ -1398,7 +1398,7 @@ public:
         levelCapacity = sink.GetRemainingCapacity();
     }
     virtual ~PartLevelCallback() {}
-    virtual UBool needToWrite(Collation::Level l) {
+    virtual UBool needToWrite(Collation::Level l) override {
         if(!sink.Overflowed()) {
             // Remember a level that will be at least partially written.
             level = l;
diff --git a/icu4c/source/i18n/strmatch.h b/icu4c/source/i18n/strmatch.h
index 4ee5cbd0887..6d2e392e654 100644
--- a/icu4c/source/i18n/strmatch.h
+++ b/icu4c/source/i18n/strmatch.h
@@ -78,21 +78,21 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
      * Implement UnicodeFunctor
      * @return a copy of the object.
      */
-    virtual StringMatcher* clone() const;
+    virtual StringMatcher* clone() const override;
 
     /**
      * UnicodeFunctor API.  Cast 'this' to a UnicodeMatcher* pointer
      * and return the pointer.
      * @return the UnicodeMatcher point.
      */
-    virtual UnicodeMatcher* toMatcher() const;
+    virtual UnicodeMatcher* toMatcher() const override;
 
     /**
      * UnicodeFunctor API.  Cast 'this' to a UnicodeReplacer* pointer
      * and return the pointer.
      * @return the UnicodeReplacer pointer.
      */
-    virtual UnicodeReplacer* toReplacer() const;
+    virtual UnicodeReplacer* toReplacer() const override;
 
     /**
      * Implement UnicodeMatcher
@@ -119,7 +119,7 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
     virtual UMatchDegree matches(const Replaceable& text,
                                  int32_t& offset,
                                  int32_t limit,
-                                 UBool incremental);
+                                 UBool incremental) override;
 
     /**
      * Implement UnicodeMatcher
@@ -128,7 +128,7 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
      * @return                  A reference to 'result'.
      */
     virtual UnicodeString& toPattern(UnicodeString& result,
-                                     UBool escapeUnprintable = false) const;
+                                     UBool escapeUnprintable = false) const override;
 
     /**
      * Implement UnicodeMatcher
@@ -140,17 +140,17 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
      * @return     true if this matcher will match a character c, 
      *             where c & 0xFF == v
      */
-    virtual UBool matchesIndexValue(uint8_t v) const;
+    virtual UBool matchesIndexValue(uint8_t v) const override;
 
     /**
      * Implement UnicodeMatcher
      */
-    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const override;
 
     /**
      * Implement UnicodeFunctor
      */
-    virtual void setData(const TransliterationRuleData*);
+    virtual void setData(const TransliterationRuleData*) override;
 
     /**
      * Replace characters in 'text' from 'start' to 'limit' with the
@@ -172,7 +172,7 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
     virtual int32_t replace(Replaceable& text,
                             int32_t start,
                             int32_t limit,
-                            int32_t& cursor);
+                            int32_t& cursor) override;
 
     /**
      * Returns a string representation of this replacer.  If the
@@ -188,7 +188,7 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
      * @return a reference to 'result'.
      */
     virtual UnicodeString& toReplacerPattern(UnicodeString& result,
-                                             UBool escapeUnprintable) const;
+                                             UBool escapeUnprintable) const override;
 
     /**
      * Remove any match data.  This must be called before performing a
@@ -199,7 +199,7 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -211,7 +211,7 @@ class StringMatcher : public UnicodeFunctor, public UnicodeMatcher, public Unico
      * into the given set.
      * @param toUnionTo the set into which to union the output characters
      */
-    virtual void addReplacementSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addReplacementSetTo(UnicodeSet& toUnionTo) const override;
 
  private:
 
diff --git a/icu4c/source/i18n/strrepl.h b/icu4c/source/i18n/strrepl.h
index 7f74d0d9450..8063626a333 100644
--- a/icu4c/source/i18n/strrepl.h
+++ b/icu4c/source/i18n/strrepl.h
@@ -111,13 +111,13 @@ class StringReplacer : public UnicodeFunctor, public UnicodeReplacer {
     /**
      * Implement UnicodeFunctor
      */
-    virtual StringReplacer* clone() const;
+    virtual StringReplacer* clone() const override;
 
     /**
      * UnicodeFunctor API.  Cast 'this' to a UnicodeReplacer* pointer
      * and return the pointer.
      */
-    virtual UnicodeReplacer* toReplacer() const;
+    virtual UnicodeReplacer* toReplacer() const override;
 
     /**
      * UnicodeReplacer API
@@ -125,23 +125,23 @@ class StringReplacer : public UnicodeFunctor, public UnicodeReplacer {
     virtual int32_t replace(Replaceable& text,
                             int32_t start,
                             int32_t limit,
-                            int32_t& cursor);
+                            int32_t& cursor) override;
 
     /**
      * UnicodeReplacer API
      */
     virtual UnicodeString& toReplacerPattern(UnicodeString& result,
-                                             UBool escapeUnprintable) const;
+                                             UBool escapeUnprintable) const override;
 
     /**
      * Implement UnicodeReplacer
      */
-    virtual void addReplacementSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addReplacementSetTo(UnicodeSet& toUnionTo) const override;
 
     /**
      * UnicodeFunctor API
      */
-    virtual void setData(const TransliterationRuleData*);
+    virtual void setData(const TransliterationRuleData*) override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -151,7 +151,7 @@ class StringReplacer : public UnicodeFunctor, public UnicodeReplacer {
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/taiwncal.h b/icu4c/source/i18n/taiwncal.h
index 5fec78d1466..ab6b6aff099 100644
--- a/icu4c/source/i18n/taiwncal.h
+++ b/icu4c/source/i18n/taiwncal.h
@@ -91,7 +91,7 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @internal
      */
-    virtual TaiwanCalendar* clone() const;
+    virtual TaiwanCalendar* clone() const override;
 
 public:
     /**
@@ -104,7 +104,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @internal
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
@@ -125,7 +125,7 @@ public:
      * @return calendar type
      * @internal
      */
-    virtual const char * getType() const;
+    virtual const char * getType() const override;
 
 private:
     TaiwanCalendar(); // default constructor not implemented
@@ -139,13 +139,13 @@ private:
      * @return the extended year
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
     /**
      * Subclasses may override this method to compute several fields
      * specific to each calendar system.  
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode& status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode& status) override;
     /**
      * Subclass API for defining limits of different types.
      * @param field one of the field numbers
@@ -153,26 +153,26 @@ private:
      * <code>LEAST_MAXIMUM</code>, or <code>MAXIMUM</code>
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
 
     /**
      * Returns true because the Taiwan Calendar does have a default century
      * @internal
      */
-    virtual UBool haveDefaultCentury() const;
+    virtual UBool haveDefaultCentury() const override;
 
     /**
      * Returns the date of the start of the default century
      * @return start of century - in milliseconds since epoch, 1970
      * @internal
      */
-    virtual UDate defaultCenturyStart() const;
+    virtual UDate defaultCenturyStart() const override;
 
     /**
      * Returns the year in which the default century begins
      * @internal
      */
-    virtual int32_t defaultCenturyStartYear() const;
+    virtual int32_t defaultCenturyStartYear() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/timezone.cpp b/icu4c/source/i18n/timezone.cpp
index 1b078a51506..ee4129e4d43 100644
--- a/icu4c/source/i18n/timezone.cpp
+++ b/icu4c/source/i18n/timezone.cpp
@@ -951,15 +951,15 @@ public:
 
     virtual ~TZEnumeration();
 
-    virtual StringEnumeration *clone() const {
+    virtual StringEnumeration *clone() const override {
         return new TZEnumeration(*this);
     }
 
-    virtual int32_t count(UErrorCode& status) const {
+    virtual int32_t count(UErrorCode& status) const override {
         return U_FAILURE(status) ? 0 : len;
     }
 
-    virtual const UnicodeString* snext(UErrorCode& status) {
+    virtual const UnicodeString* snext(UErrorCode& status) override {
         if (U_SUCCESS(status) && map != NULL && pos < len) {
             getID(map[pos], status);
             ++pos;
@@ -968,13 +968,13 @@ public:
         return 0;
     }
 
-    virtual void reset(UErrorCode& /*status*/) {
+    virtual void reset(UErrorCode& /*status*/) override {
         pos = 0;
     }
 
 public:
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 TZEnumeration::~TZEnumeration() {
diff --git a/icu4c/source/i18n/titletrn.h b/icu4c/source/i18n/titletrn.h
index 4e45ac6f81f..8409519818a 100644
--- a/icu4c/source/i18n/titletrn.h
+++ b/icu4c/source/i18n/titletrn.h
@@ -52,12 +52,12 @@ class TitlecaseTransliterator : public CaseMapTransliterator {
      * Transliterator API.
      * @return a copy of the object.
      */
-    virtual TitlecaseTransliterator* clone() const;
+    virtual TitlecaseTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -76,7 +76,7 @@ protected:
      *                    pos.contextLimit.  Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                             UBool isIncremental) const;
+                             UBool isIncremental) const override;
 
 private:
     /**
diff --git a/icu4c/source/i18n/tmutfmt.cpp b/icu4c/source/i18n/tmutfmt.cpp
index 6068a0189c8..057bb634ebb 100644
--- a/icu4c/source/i18n/tmutfmt.cpp
+++ b/icu4c/source/i18n/tmutfmt.cpp
@@ -362,7 +362,7 @@ struct TimeUnitFormatReadSink : public ResourceSink {
 
     virtual ~TimeUnitFormatReadSink();
 
-    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) {
+    virtual void put(const char *key, ResourceValue &value, UBool, UErrorCode &errorCode) override {
         // Skip all put() calls except the first one -- discard all fallback data.
         if (beenHere) {
             return;
diff --git a/icu4c/source/i18n/tolowtrn.h b/icu4c/source/i18n/tolowtrn.h
index 2fbfb90e1bf..951128ec77e 100644
--- a/icu4c/source/i18n/tolowtrn.h
+++ b/icu4c/source/i18n/tolowtrn.h
@@ -50,12 +50,12 @@ class LowercaseTransliterator : public CaseMapTransliterator {
      * Transliterator API.
      * @return a copy of the object.
      */
-    virtual LowercaseTransliterator* clone() const;
+    virtual LowercaseTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/toupptrn.h b/icu4c/source/i18n/toupptrn.h
index e96ca8f0ba5..755e9edfcb5 100644
--- a/icu4c/source/i18n/toupptrn.h
+++ b/icu4c/source/i18n/toupptrn.h
@@ -50,12 +50,12 @@ class UppercaseTransliterator : public CaseMapTransliterator {
      * Transliterator API.
      * @return a copy of the object.
      */
-    virtual UppercaseTransliterator* clone() const;
+    virtual UppercaseTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/transreg.h b/icu4c/source/i18n/transreg.h
index 18deec85156..686e62ae33c 100644
--- a/icu4c/source/i18n/transreg.h
+++ b/icu4c/source/i18n/transreg.h
@@ -417,11 +417,11 @@ class TransliteratorRegistry : public UMemory {
     public:
         Enumeration(const TransliteratorRegistry& reg);
         virtual ~Enumeration();
-        virtual int32_t count(UErrorCode& status) const;
-        virtual const UnicodeString* snext(UErrorCode& status);
-        virtual void reset(UErrorCode& status);
+        virtual int32_t count(UErrorCode& status) const override;
+        virtual const UnicodeString* snext(UErrorCode& status) override;
+        virtual void reset(UErrorCode& status) override;
         static UClassID U_EXPORT2 getStaticClassID();
-        virtual UClassID getDynamicClassID() const;
+        virtual UClassID getDynamicClassID() const override;
     private:
         int32_t index;
         const TransliteratorRegistry& reg;
diff --git a/icu4c/source/i18n/tzfmt.cpp b/icu4c/source/i18n/tzfmt.cpp
index c16d4131682..38947a79aa3 100644
--- a/icu4c/source/i18n/tzfmt.cpp
+++ b/icu4c/source/i18n/tzfmt.cpp
@@ -2729,7 +2729,7 @@ public:
     ZoneIdMatchHandler();
     virtual ~ZoneIdMatchHandler();
 
-    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status);
+    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status) override;
     const UChar* getID();
     int32_t getMatchLen();
 private:
diff --git a/icu4c/source/i18n/tzgnames.cpp b/icu4c/source/i18n/tzgnames.cpp
index bab3eb353f6..ed5f42d7bc1 100644
--- a/icu4c/source/i18n/tzgnames.cpp
+++ b/icu4c/source/i18n/tzgnames.cpp
@@ -195,7 +195,7 @@ public:
     GNameSearchHandler(uint32_t types);
     virtual ~GNameSearchHandler();
 
-    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status);
+    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status) override;
     UVector* getMatches(int32_t& maxMatchLen);
 
 private:
diff --git a/icu4c/source/i18n/tznames.cpp b/icu4c/source/i18n/tznames.cpp
index 796ba73de00..5c504d01cb6 100644
--- a/icu4c/source/i18n/tznames.cpp
+++ b/icu4c/source/i18n/tznames.cpp
@@ -104,24 +104,24 @@ public:
     TimeZoneNamesDelegate(const Locale& locale, UErrorCode& status);
     virtual ~TimeZoneNamesDelegate();
 
-    virtual bool operator==(const TimeZoneNames& other) const;
+    virtual bool operator==(const TimeZoneNames& other) const override;
     virtual bool operator!=(const TimeZoneNames& other) const {return !operator==(other);}
-    virtual TimeZoneNamesDelegate* clone() const;
+    virtual TimeZoneNamesDelegate* clone() const override;
 
-    StringEnumeration* getAvailableMetaZoneIDs(UErrorCode& status) const;
-    StringEnumeration* getAvailableMetaZoneIDs(const UnicodeString& tzID, UErrorCode& status) const;
-    UnicodeString& getMetaZoneID(const UnicodeString& tzID, UDate date, UnicodeString& mzID) const;
-    UnicodeString& getReferenceZoneID(const UnicodeString& mzID, const char* region, UnicodeString& tzID) const;
+    StringEnumeration* getAvailableMetaZoneIDs(UErrorCode& status) const override;
+    StringEnumeration* getAvailableMetaZoneIDs(const UnicodeString& tzID, UErrorCode& status) const override;
+    UnicodeString& getMetaZoneID(const UnicodeString& tzID, UDate date, UnicodeString& mzID) const override;
+    UnicodeString& getReferenceZoneID(const UnicodeString& mzID, const char* region, UnicodeString& tzID) const override;
 
-    UnicodeString& getMetaZoneDisplayName(const UnicodeString& mzID, UTimeZoneNameType type, UnicodeString& name) const;
-    UnicodeString& getTimeZoneDisplayName(const UnicodeString& tzID, UTimeZoneNameType type, UnicodeString& name) const;
+    UnicodeString& getMetaZoneDisplayName(const UnicodeString& mzID, UTimeZoneNameType type, UnicodeString& name) const override;
+    UnicodeString& getTimeZoneDisplayName(const UnicodeString& tzID, UTimeZoneNameType type, UnicodeString& name) const override;
 
-    UnicodeString& getExemplarLocationName(const UnicodeString& tzID, UnicodeString& name) const;
+    UnicodeString& getExemplarLocationName(const UnicodeString& tzID, UnicodeString& name) const override;
 
-    void loadAllDisplayNames(UErrorCode& status);
-    void getDisplayNames(const UnicodeString& tzID, const UTimeZoneNameType types[], int32_t numTypes, UDate date, UnicodeString dest[], UErrorCode& status) const;
+    void loadAllDisplayNames(UErrorCode& status) override;
+    void getDisplayNames(const UnicodeString& tzID, const UTimeZoneNameType types[], int32_t numTypes, UDate date, UnicodeString dest[], UErrorCode& status) const override;
 
-    MatchInfoCollection* find(const UnicodeString& text, int32_t start, uint32_t types, UErrorCode& status) const;
+    MatchInfoCollection* find(const UnicodeString& text, int32_t start, uint32_t types, UErrorCode& status) const override;
 private:
     TimeZoneNamesDelegate();
     TimeZoneNamesCacheEntry*    fTZnamesCacheEntry;
diff --git a/icu4c/source/i18n/tznames_impl.cpp b/icu4c/source/i18n/tznames_impl.cpp
index d6bb25d0ea0..d450b745648 100644
--- a/icu4c/source/i18n/tznames_impl.cpp
+++ b/icu4c/source/i18n/tznames_impl.cpp
@@ -794,7 +794,7 @@ struct ZNames::ZNamesLoader : public ResourceSink {
     }
 
     virtual void put(const char* key, ResourceValue& value, UBool /*noFallback*/,
-            UErrorCode &errorCode) {
+            UErrorCode &errorCode) override {
         ResourceTable namesTable = value.getTable(errorCode);
         if (U_FAILURE(errorCode)) { return; }
         for (int32_t i = 0; namesTable.getKeyAndValue(i, key, value); ++i) {
@@ -857,10 +857,10 @@ public:
     MetaZoneIDsEnumeration(UVector* mzIDs);
     virtual ~MetaZoneIDsEnumeration();
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
-    virtual const UnicodeString* snext(UErrorCode& status);
-    virtual void reset(UErrorCode& status);
-    virtual int32_t count(UErrorCode& status) const;
+    virtual UClassID getDynamicClassID(void) const override;
+    virtual const UnicodeString* snext(UErrorCode& status) override;
+    virtual void reset(UErrorCode& status) override;
+    virtual int32_t count(UErrorCode& status) const override;
 private:
     int32_t fLen;
     int32_t fPos;
@@ -920,7 +920,7 @@ public:
     ZNameSearchHandler(uint32_t types);
     virtual ~ZNameSearchHandler();
 
-    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status);
+    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status) override;
     TimeZoneNames::MatchInfoCollection* getMatches(int32_t& maxMatchLen);
 
 private:
@@ -1560,7 +1560,7 @@ struct TimeZoneNamesImpl::ZoneStringsLoader : public ResourceSink {
     }
 
     virtual void put(const char *key, ResourceValue &value, UBool noFallback,
-            UErrorCode &status) {
+            UErrorCode &status) override {
         ResourceTable timeZonesTable = value.getTable(status);
         if (U_FAILURE(status)) { return; }
         for (int32_t i = 0; timeZonesTable.getKeyAndValue(i, key, value); ++i) {
@@ -1891,7 +1891,7 @@ public:
     TZDBNameSearchHandler(uint32_t types, const char* region);
     virtual ~TZDBNameSearchHandler();
 
-    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status);
+    UBool handleMatch(int32_t matchLength, const CharacterNode *node, UErrorCode &status) override;
     TimeZoneNames::MatchInfoCollection* getMatches(int32_t& maxMatchLen);
 
 private:
diff --git a/icu4c/source/i18n/tznames_impl.h b/icu4c/source/i18n/tznames_impl.h
index e68b198dba7..d047fa35418 100644
--- a/icu4c/source/i18n/tznames_impl.h
+++ b/icu4c/source/i18n/tznames_impl.h
@@ -173,24 +173,24 @@ public:
 
     virtual ~TimeZoneNamesImpl();
 
-    virtual bool operator==(const TimeZoneNames& other) const;
-    virtual TimeZoneNamesImpl* clone() const;
+    virtual bool operator==(const TimeZoneNames& other) const override;
+    virtual TimeZoneNamesImpl* clone() const override;
 
-    StringEnumeration* getAvailableMetaZoneIDs(UErrorCode& status) const;
-    StringEnumeration* getAvailableMetaZoneIDs(const UnicodeString& tzID, UErrorCode& status) const;
+    StringEnumeration* getAvailableMetaZoneIDs(UErrorCode& status) const override;
+    StringEnumeration* getAvailableMetaZoneIDs(const UnicodeString& tzID, UErrorCode& status) const override;
 
-    UnicodeString& getMetaZoneID(const UnicodeString& tzID, UDate date, UnicodeString& mzID) const;
-    UnicodeString& getReferenceZoneID(const UnicodeString& mzID, const char* region, UnicodeString& tzID) const;
+    UnicodeString& getMetaZoneID(const UnicodeString& tzID, UDate date, UnicodeString& mzID) const override;
+    UnicodeString& getReferenceZoneID(const UnicodeString& mzID, const char* region, UnicodeString& tzID) const override;
 
-    UnicodeString& getMetaZoneDisplayName(const UnicodeString& mzID, UTimeZoneNameType type, UnicodeString& name) const;
-    UnicodeString& getTimeZoneDisplayName(const UnicodeString& tzID, UTimeZoneNameType type, UnicodeString& name) const;
+    UnicodeString& getMetaZoneDisplayName(const UnicodeString& mzID, UTimeZoneNameType type, UnicodeString& name) const override;
+    UnicodeString& getTimeZoneDisplayName(const UnicodeString& tzID, UTimeZoneNameType type, UnicodeString& name) const override;
 
-    UnicodeString& getExemplarLocationName(const UnicodeString& tzID, UnicodeString& name) const;
+    UnicodeString& getExemplarLocationName(const UnicodeString& tzID, UnicodeString& name) const override;
 
-    TimeZoneNames::MatchInfoCollection* find(const UnicodeString& text, int32_t start, uint32_t types, UErrorCode& status) const;
+    TimeZoneNames::MatchInfoCollection* find(const UnicodeString& text, int32_t start, uint32_t types, UErrorCode& status) const override;
 
-    void loadAllDisplayNames(UErrorCode& status);
-    void getDisplayNames(const UnicodeString& tzID, const UTimeZoneNameType types[], int32_t numTypes, UDate date, UnicodeString dest[], UErrorCode& status) const;
+    void loadAllDisplayNames(UErrorCode& status) override;
+    void getDisplayNames(const UnicodeString& tzID, const UTimeZoneNameType types[], int32_t numTypes, UDate date, UnicodeString dest[], UErrorCode& status) const override;
 
     static UnicodeString& getDefaultExemplarLocationName(const UnicodeString& tzID, UnicodeString& name);
 
@@ -235,19 +235,19 @@ public:
     TZDBTimeZoneNames(const Locale& locale);
     virtual ~TZDBTimeZoneNames();
 
-    virtual bool operator==(const TimeZoneNames& other) const;
-    virtual TZDBTimeZoneNames* clone() const;
+    virtual bool operator==(const TimeZoneNames& other) const override;
+    virtual TZDBTimeZoneNames* clone() const override;
 
-    StringEnumeration* getAvailableMetaZoneIDs(UErrorCode& status) const;
-    StringEnumeration* getAvailableMetaZoneIDs(const UnicodeString& tzID, UErrorCode& status) const;
+    StringEnumeration* getAvailableMetaZoneIDs(UErrorCode& status) const override;
+    StringEnumeration* getAvailableMetaZoneIDs(const UnicodeString& tzID, UErrorCode& status) const override;
 
-    UnicodeString& getMetaZoneID(const UnicodeString& tzID, UDate date, UnicodeString& mzID) const;
-    UnicodeString& getReferenceZoneID(const UnicodeString& mzID, const char* region, UnicodeString& tzID) const;
+    UnicodeString& getMetaZoneID(const UnicodeString& tzID, UDate date, UnicodeString& mzID) const override;
+    UnicodeString& getReferenceZoneID(const UnicodeString& mzID, const char* region, UnicodeString& tzID) const override;
 
-    UnicodeString& getMetaZoneDisplayName(const UnicodeString& mzID, UTimeZoneNameType type, UnicodeString& name) const;
-    UnicodeString& getTimeZoneDisplayName(const UnicodeString& tzID, UTimeZoneNameType type, UnicodeString& name) const;
+    UnicodeString& getMetaZoneDisplayName(const UnicodeString& mzID, UTimeZoneNameType type, UnicodeString& name) const override;
+    UnicodeString& getTimeZoneDisplayName(const UnicodeString& tzID, UTimeZoneNameType type, UnicodeString& name) const override;
 
-    TimeZoneNames::MatchInfoCollection* find(const UnicodeString& text, int32_t start, uint32_t types, UErrorCode& status) const;
+    TimeZoneNames::MatchInfoCollection* find(const UnicodeString& text, int32_t start, uint32_t types, UErrorCode& status) const override;
 
     // When TZDBNames for the metazone is not available, this method returns NULL,
     // but does NOT set U_MISSING_RESOURCE_ERROR to status.
diff --git a/icu4c/source/i18n/ucol_res.cpp b/icu4c/source/i18n/ucol_res.cpp
index aa4027eb872..b277cf3b280 100644
--- a/icu4c/source/i18n/ucol_res.cpp
+++ b/icu4c/source/i18n/ucol_res.cpp
@@ -623,7 +623,7 @@ public:
     virtual ~KeywordsSink();
 
     virtual void put(const char *key, ResourceValue &value, UBool /*noFallback*/,
-                     UErrorCode &errorCode) {
+                     UErrorCode &errorCode) override {
         if (U_FAILURE(errorCode)) { return; }
         ResourceTable collations = value.getTable(errorCode);
         for (int32_t i = 0; collations.getKeyAndValue(i, key, value); ++i) {
diff --git a/icu4c/source/i18n/uitercollationiterator.h b/icu4c/source/i18n/uitercollationiterator.h
index 3a7b1a0ec23..e0da353f268 100644
--- a/icu4c/source/i18n/uitercollationiterator.h
+++ b/icu4c/source/i18n/uitercollationiterator.h
@@ -39,22 +39,22 @@ public:
 
     virtual ~UIterCollationIterator();
 
-    virtual void resetToOffset(int32_t newOffset);
+    virtual void resetToOffset(int32_t newOffset) override;
 
-    virtual int32_t getOffset() const;
+    virtual int32_t getOffset() const override;
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
 
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
-    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode);
+    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode) override;
 
-    virtual UChar handleGetTrailSurrogate();
+    virtual UChar handleGetTrailSurrogate() override;
 
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
     UCharIterator &iter;
 };
@@ -71,22 +71,23 @@ public:
 
     virtual ~FCDUIterCollationIterator();
 
-    virtual void resetToOffset(int32_t newOffset);
+    virtual void resetToOffset(int32_t newOffset) override;
 
-    virtual int32_t getOffset() const;
+    virtual int32_t getOffset() const override;
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
 
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
-    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode);
+    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode) override;
 
-    virtual UChar handleGetTrailSurrogate();
+    virtual UChar handleGetTrailSurrogate() override;
 
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
 
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
+
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
 private:
     /**
diff --git a/icu4c/source/i18n/unesctrn.h b/icu4c/source/i18n/unesctrn.h
index 57dd8d32cf5..e905c5bc5f5 100644
--- a/icu4c/source/i18n/unesctrn.h
+++ b/icu4c/source/i18n/unesctrn.h
@@ -77,12 +77,12 @@ class UnescapeTransliterator : public Transliterator {
     /**
      * Transliterator API.
      */
-    virtual UnescapeTransliterator* clone() const;
+    virtual UnescapeTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -101,7 +101,7 @@ class UnescapeTransliterator : public Transliterator {
      *                    pos.contextLimit.  Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                             UBool isIncremental) const;
+                             UBool isIncremental) const override;
 
 };
 
diff --git a/icu4c/source/i18n/uni2name.h b/icu4c/source/i18n/uni2name.h
index 99309c8e0f8..1e01d784237 100644
--- a/icu4c/source/i18n/uni2name.h
+++ b/icu4c/source/i18n/uni2name.h
@@ -48,12 +48,12 @@ class UnicodeNameTransliterator : public Transliterator {
     /**
      * Transliterator API.
      */
-    virtual UnicodeNameTransliterator* clone() const;
+    virtual UnicodeNameTransliterator* clone() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -72,7 +72,7 @@ class UnicodeNameTransliterator : public Transliterator {
      *                    pos.contextLimit.  Otherwise, assume the text is complete.
      */
     virtual void handleTransliterate(Replaceable& text, UTransPosition& offset,
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
 private:
     /**
diff --git a/icu4c/source/i18n/unicode/choicfmt.h b/icu4c/source/i18n/unicode/choicfmt.h
index 46b84f4f5b2..0e7ae186d4d 100644
--- a/icu4c/source/i18n/unicode/choicfmt.h
+++ b/icu4c/source/i18n/unicode/choicfmt.h
@@ -251,7 +251,7 @@ public:
      * @return a copy of this object
      * @deprecated ICU 49 Use MessageFormat instead, with plural and select arguments.
      */
-    virtual ChoiceFormat* clone() const;
+    virtual ChoiceFormat* clone() const override;
 
     /**
      * Returns true if the given Format objects are semantically equal.
@@ -261,7 +261,7 @@ public:
      * @return         true if other is the same as this.
      * @deprecated ICU 49 Use MessageFormat instead, with plural and select arguments.
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
     /**
      * Sets the pattern.
@@ -375,7 +375,7 @@ public:
      */
     virtual UnicodeString& format(double number,
                                   UnicodeString& appendTo,
-                                  FieldPosition& pos) const;
+                                  FieldPosition& pos) const override;
     /**
      * Formats an int32_t number using this object's choices.
      *
@@ -389,7 +389,7 @@ public:
      */
     virtual UnicodeString& format(int32_t number,
                                   UnicodeString& appendTo,
-                                  FieldPosition& pos) const;
+                                  FieldPosition& pos) const override;
 
     /**
      * Formats an int64_t number using this object's choices.
@@ -404,7 +404,7 @@ public:
      */
     virtual UnicodeString& format(int64_t number,
                                   UnicodeString& appendTo,
-                                  FieldPosition& pos) const;
+                                  FieldPosition& pos) const override;
 
     /**
      * Formats an array of objects using this object's choices.
@@ -444,7 +444,7 @@ public:
     */
     virtual void parse(const UnicodeString& text,
                        Formattable& result,
-                       ParsePosition& parsePosition) const;
+                       ParsePosition& parsePosition) const override;
 
     /**
      * Returns a unique class ID POLYMORPHICALLY. Part of ICU's "poor man's RTTI".
@@ -454,7 +454,7 @@ public:
      *                  other classes have different class IDs.
      * @deprecated ICU 49 Use MessageFormat instead, with plural and select arguments.
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Returns the class ID for this class.  This is useful only for
diff --git a/icu4c/source/i18n/unicode/coleitr.h b/icu4c/source/i18n/unicode/coleitr.h
index 38a63d23df6..4e68c6f1749 100644
--- a/icu4c/source/i18n/unicode/coleitr.h
+++ b/icu4c/source/i18n/unicode/coleitr.h
@@ -279,7 +279,7 @@ public:
     *
     * @stable ICU 2.2
     */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
     * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/curramt.h b/icu4c/source/i18n/unicode/curramt.h
index e4c35c18111..818b11ae5bd 100644
--- a/icu4c/source/i18n/unicode/curramt.h
+++ b/icu4c/source/i18n/unicode/curramt.h
@@ -81,7 +81,7 @@ class U_I18N_API CurrencyAmount: public Measure {
      * have the same class as returned by getDynamicClassID().
      * @stable ICU 3.0
      */
-    virtual CurrencyAmount* clone() const;
+    virtual CurrencyAmount* clone() const override;
 
     /**
      * Destructor
@@ -97,7 +97,7 @@ class U_I18N_API CurrencyAmount: public Measure {
      * different class IDs.
      * @stable ICU 3.0
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * Returns the class ID for this class. This is used to compare to
diff --git a/icu4c/source/i18n/unicode/currpinf.h b/icu4c/source/i18n/unicode/currpinf.h
index 8e73f9f00ab..e3ca34b26f2 100644
--- a/icu4c/source/i18n/unicode/currpinf.h
+++ b/icu4c/source/i18n/unicode/currpinf.h
@@ -184,7 +184,7 @@ public:
      *
      * @stable ICU 4.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/currunit.h b/icu4c/source/i18n/unicode/currunit.h
index 9b608fdeef2..8da3f1ffd87 100644
--- a/icu4c/source/i18n/unicode/currunit.h
+++ b/icu4c/source/i18n/unicode/currunit.h
@@ -94,7 +94,7 @@ class U_I18N_API CurrencyUnit: public MeasureUnit {
      * have the same class as returned by getDynamicClassID().
      * @stable ICU 3.0
      */
-    virtual CurrencyUnit* clone() const;
+    virtual CurrencyUnit* clone() const override;
 
     /**
      * Destructor
@@ -110,7 +110,7 @@ class U_I18N_API CurrencyUnit: public MeasureUnit {
      * different class IDs.
      * @stable ICU 3.0
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * Returns the class ID for this class. This is used to compare to
diff --git a/icu4c/source/i18n/unicode/dcfmtsym.h b/icu4c/source/i18n/unicode/dcfmtsym.h
index 5b924458fa4..3b1535dd16d 100644
--- a/icu4c/source/i18n/unicode/dcfmtsym.h
+++ b/icu4c/source/i18n/unicode/dcfmtsym.h
@@ -359,7 +359,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/dtfmtsym.h b/icu4c/source/i18n/unicode/dtfmtsym.h
index 3c4756c4d7f..200b3a6ef17 100644
--- a/icu4c/source/i18n/unicode/dtfmtsym.h
+++ b/icu4c/source/i18n/unicode/dtfmtsym.h
@@ -628,7 +628,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/dtitvfmt.h b/icu4c/source/i18n/unicode/dtitvfmt.h
index e258a380e3e..85a1b365d83 100644
--- a/icu4c/source/i18n/unicode/dtitvfmt.h
+++ b/icu4c/source/i18n/unicode/dtitvfmt.h
@@ -434,7 +434,7 @@ public:
      * @return    A copy of the object.
      * @stable ICU 4.0
      */
-    virtual DateIntervalFormat* clone() const;
+    virtual DateIntervalFormat* clone() const override;
 
     /**
      * Return true if the given Format objects are semantically equal. Objects
@@ -443,7 +443,7 @@ public:
      * @return         true if the given Format objects are semantically equal.
      * @stable ICU 4.0
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
     /**
      * Return true if the given Format objects are not semantically equal.
@@ -479,7 +479,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPosition& fieldPosition,
-                                  UErrorCode& status) const ;
+                                  UErrorCode& status) const override;
 
 
 
@@ -595,7 +595,7 @@ public:
      */
     virtual void parseObject(const UnicodeString& source,
                              Formattable& result,
-                             ParsePosition& parse_pos) const;
+                             ParsePosition& parse_pos) const override;
 
 
     /**
@@ -702,7 +702,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 4.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
 protected:
 
diff --git a/icu4c/source/i18n/unicode/dtitvinf.h b/icu4c/source/i18n/unicode/dtitvinf.h
index f6368c4f790..b5afc064678 100644
--- a/icu4c/source/i18n/unicode/dtitvinf.h
+++ b/icu4c/source/i18n/unicode/dtitvinf.h
@@ -319,7 +319,7 @@ public:
      *
      * @stable ICU 4.0
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/dtptngen.h b/icu4c/source/i18n/unicode/dtptngen.h
index ce0431c7264..250a0e089fe 100644
--- a/icu4c/source/i18n/unicode/dtptngen.h
+++ b/icu4c/source/i18n/unicode/dtptngen.h
@@ -513,7 +513,7 @@ public:
      *
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/dtrule.h b/icu4c/source/i18n/unicode/dtrule.h
index 4133efc5079..19e94bc981c 100644
--- a/icu4c/source/i18n/unicode/dtrule.h
+++ b/icu4c/source/i18n/unicode/dtrule.h
@@ -243,7 +243,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/unicode/fieldpos.h b/icu4c/source/i18n/unicode/fieldpos.h
index d31fcf3888e..b985f3b2f76 100644
--- a/icu4c/source/i18n/unicode/fieldpos.h
+++ b/icu4c/source/i18n/unicode/fieldpos.h
@@ -236,7 +236,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/fmtable.h b/icu4c/source/i18n/unicode/fmtable.h
index e3764df96c2..e1c6dec35e4 100644
--- a/icu4c/source/i18n/unicode/fmtable.h
+++ b/icu4c/source/i18n/unicode/fmtable.h
@@ -587,7 +587,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/gregocal.h b/icu4c/source/i18n/unicode/gregocal.h
index 4c79944a6ff..4c2be18251f 100644
--- a/icu4c/source/i18n/unicode/gregocal.h
+++ b/icu4c/source/i18n/unicode/gregocal.h
@@ -303,7 +303,7 @@ public:
      * @return    return a polymorphic copy of this calendar.
      * @stable ICU 2.0
      */
-    virtual GregorianCalendar* clone() const;
+    virtual GregorianCalendar* clone() const override;
 
     /**
      * Sets the GregorianCalendar change date. This is the point when the switch from
@@ -350,7 +350,7 @@ public:
      * @param other the Calendar to be compared with this Calendar   
      * @stable ICU 2.4
      */
-    virtual UBool isEquivalentTo(const Calendar& other) const;
+    virtual UBool isEquivalentTo(const Calendar& other) const override;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
@@ -364,7 +364,7 @@ public:
      *                an error status.
      * @deprecated ICU 2.6. Use roll(UCalendarDateFields field, int32_t amount, UErrorCode& status) instead.
      */
-    virtual void roll(EDateFields field, int32_t amount, UErrorCode& status);
+    virtual void roll(EDateFields field, int32_t amount, UErrorCode& status) override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -378,7 +378,7 @@ public:
      *                an error status.
      * @stable ICU 2.6.
      */
-    virtual void roll(UCalendarDateFields field, int32_t amount, UErrorCode& status);
+    virtual void roll(UCalendarDateFields field, int32_t amount, UErrorCode& status) override;
 
 #ifndef U_HIDE_DEPRECATED_API
     /**
@@ -409,7 +409,7 @@ public:
      * @return         the minimum value that this field could have, given the current date.
      * @stable ICU 3.0
      */
-    int32_t getActualMinimum(UCalendarDateFields field, UErrorCode &status) const;
+    int32_t getActualMinimum(UCalendarDateFields field, UErrorCode &status) const override;
 
 #ifndef U_HIDE_DEPRECATED_API
     /**
@@ -434,7 +434,7 @@ public:
      * @return         the maximum value that this field could have, given the current date.
      * @stable ICU 2.6
      */
-    virtual int32_t getActualMaximum(UCalendarDateFields field, UErrorCode& status) const;
+    virtual int32_t getActualMaximum(UCalendarDateFields field, UErrorCode& status) const override;
 
     /**
      * (Overrides Calendar) Return true if the current date for this Calendar is in
@@ -445,7 +445,7 @@ public:
      *           false, otherwise.
      * @stable ICU 2.0
      */
-    virtual UBool inDaylightTime(UErrorCode& status) const;
+    virtual UBool inDaylightTime(UErrorCode& status) const override;
 
 public:
 
@@ -459,7 +459,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
@@ -484,7 +484,7 @@ public:
      * @return legacy calendar type name string
      * @stable ICU 49
      */
-    virtual const char * getType() const;
+    virtual const char * getType() const override;
 
  private:
     GregorianCalendar(); // default constructor not implemented
@@ -512,7 +512,7 @@ public:
      * @internal
      */
     virtual int32_t handleComputeMonthStart(int32_t eyear, int32_t month,
-                                                   UBool useMonth) const;
+                                                   UBool useMonth) const override;
 
     /**
      * Subclasses may override this.  This method calls
@@ -522,7 +522,7 @@ public:
      * @return julian day specified by calendar fields.
      * @internal
      */
-    virtual int32_t handleComputeJulianDay(UCalendarDateFields bestField)  ;
+    virtual int32_t handleComputeJulianDay(UCalendarDateFields bestField) override;
 
     /**
      * Return the number of days in the given month of the given extended
@@ -531,7 +531,7 @@ public:
      * implementation than the default implementation in Calendar.
      * @internal
      */
-    virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const;
+    virtual int32_t handleGetMonthLength(int32_t extendedYear, int32_t month) const override;
 
     /**
      * Return the number of days in the given extended year of this
@@ -540,7 +540,7 @@ public:
      * default implementation in Calendar.
      * @stable ICU 2.0
      */
-    virtual int32_t handleGetYearLength(int32_t eyear) const;
+    virtual int32_t handleGetYearLength(int32_t eyear) const override;
 
     /**
      * return the length of the given month.
@@ -615,7 +615,7 @@ public:
      * <code>LEAST_MAXIMUM</code>, or <code>MAXIMUM</code>
      * @internal
      */
-    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const;
+    virtual int32_t handleGetLimit(UCalendarDateFields field, ELimitType limitType) const override;
 
     /**
      * Return the extended year defined by the current fields.  This will
@@ -625,7 +625,7 @@ public:
      * @return the extended year
      * @internal
      */
-    virtual int32_t handleGetExtendedYear();
+    virtual int32_t handleGetExtendedYear() override;
 
     /** 
      * Subclasses may override this to convert from week fields 
@@ -635,7 +635,7 @@ public:
      * @return the extended year, UCAL_EXTENDED_YEAR
      * @internal
      */
-    virtual int32_t handleGetExtendedYearFromWeekFields(int32_t yearWoy, int32_t woy);
+    virtual int32_t handleGetExtendedYearFromWeekFields(int32_t yearWoy, int32_t woy) override;
 
 
     /**
@@ -653,7 +653,7 @@ public:
      * a calendar with the specified Julian/Gregorian cutover date.
      * @internal
      */
-    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status);
+    virtual void handleComputeFields(int32_t julianDay, UErrorCode &status) override;
 
  private:
     /**
@@ -759,19 +759,19 @@ public:
      * @return true if this calendar has the notion of a default century
      * @internal 
      */
-    virtual UBool haveDefaultCentury() const;
+    virtual UBool haveDefaultCentury() const override;
 
     /**
      * @return the start of the default century
      * @internal
      */
-    virtual UDate defaultCenturyStart() const;
+    virtual UDate defaultCenturyStart() const override;
 
     /**
      * @return the beginning year of the default century
      * @internal 
      */
-    virtual int32_t defaultCenturyStartYear() const;
+    virtual int32_t defaultCenturyStartYear() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/unicode/measfmt.h b/icu4c/source/i18n/unicode/measfmt.h
index 779212cf1d1..893076145f5 100644
--- a/icu4c/source/i18n/unicode/measfmt.h
+++ b/icu4c/source/i18n/unicode/measfmt.h
@@ -148,13 +148,13 @@ class U_I18N_API MeasureFormat : public Format {
      * Return true if given Format objects are semantically equal.
      * @stable ICU 53
      */
-    virtual bool operator==(const Format &other) const;
+    virtual bool operator==(const Format &other) const override;
 
     /**
      * Clones this object polymorphically.
      * @stable ICU 53
      */
-    virtual MeasureFormat *clone() const;
+    virtual MeasureFormat *clone() const override;
 
     /**
      * Formats object to produce a string.
@@ -164,7 +164,7 @@ class U_I18N_API MeasureFormat : public Format {
             const Formattable &obj,
             UnicodeString &appendTo,
             FieldPosition &pos,
-            UErrorCode &status) const;
+            UErrorCode &status) const override;
 
 #ifndef U_FORCE_HIDE_DRAFT_API
     /**
@@ -176,7 +176,7 @@ class U_I18N_API MeasureFormat : public Format {
     virtual void parseObject(
             const UnicodeString &source,
             Formattable &reslt,
-            ParsePosition &pos) const;
+            ParsePosition &pos) const override;
 #endif  // U_FORCE_HIDE_DRAFT_API
 
     /**
@@ -286,7 +286,7 @@ class U_I18N_API MeasureFormat : public Format {
      *                  other classes have different class IDs.
      * @stable ICU 53
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
  protected:
     /**
diff --git a/icu4c/source/i18n/unicode/measunit.h b/icu4c/source/i18n/unicode/measunit.h
index be6688410d6..d0b70212fcb 100644
--- a/icu4c/source/i18n/unicode/measunit.h
+++ b/icu4c/source/i18n/unicode/measunit.h
@@ -662,7 +662,7 @@ class U_I18N_API MeasureUnit: public UObject {
      *                  other classes have different class IDs.
      * @stable ICU 53
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
 #ifndef U_HIDE_INTERNAL_API
     /**
diff --git a/icu4c/source/i18n/unicode/measure.h b/icu4c/source/i18n/unicode/measure.h
index 3831e9dd2f1..7b118acfcfa 100644
--- a/icu4c/source/i18n/unicode/measure.h
+++ b/icu4c/source/i18n/unicode/measure.h
@@ -127,7 +127,7 @@ class U_I18N_API Measure: public UObject {
      *                  other classes have different class IDs.
      * @stable ICU 53
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
  protected:
     /**
diff --git a/icu4c/source/i18n/unicode/msgfmt.h b/icu4c/source/i18n/unicode/msgfmt.h
index cb2c66e8f9d..c949132fdae 100644
--- a/icu4c/source/i18n/unicode/msgfmt.h
+++ b/icu4c/source/i18n/unicode/msgfmt.h
@@ -420,7 +420,7 @@ public:
      * result and should delete it when done.
      * @stable ICU 2.0
      */
-    virtual MessageFormat* clone() const;
+    virtual MessageFormat* clone() const override;
 
     /**
      * Returns true if the given Format objects are semantically equal.
@@ -429,7 +429,7 @@ public:
      * @return       true if the given Format objects are semantically equal.
      * @stable ICU 2.0
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
     /**
      * Sets the locale to be used for creating argument Format objects.
@@ -715,7 +715,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPosition& pos,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
 
     /**
      * Formats the given array of arguments into a user-defined argument name
@@ -790,7 +790,7 @@ public:
      */
     virtual void parseObject(const UnicodeString& source,
                              Formattable& result,
-                             ParsePosition& pos) const;
+                             ParsePosition& pos) const override;
 
     /**
      * Convert an 'apostrophe-friendly' pattern into a standard
@@ -850,7 +850,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class.  This is useful only for
@@ -898,7 +898,7 @@ private:
     public:
         PluralSelectorProvider(const MessageFormat &mf, UPluralType type);
         virtual ~PluralSelectorProvider();
-        virtual UnicodeString select(void *ctx, double number, UErrorCode& ec) const;
+        virtual UnicodeString select(void *ctx, double number, UErrorCode& ec) const override;
 
         void reset();
     private:
@@ -1088,22 +1088,22 @@ private:
      */
     class U_I18N_API DummyFormat : public Format {
     public:
-        virtual bool operator==(const Format&) const;
-        virtual DummyFormat* clone() const;
+        virtual bool operator==(const Format&) const override;
+        virtual DummyFormat* clone() const override;
         virtual UnicodeString& format(const Formattable& obj,
                               UnicodeString& appendTo,
                               UErrorCode& status) const;
         virtual UnicodeString& format(const Formattable&,
                                       UnicodeString& appendTo,
                                       FieldPosition&,
-                                      UErrorCode& status) const;
+                                      UErrorCode& status) const override;
         virtual UnicodeString& format(const Formattable& obj,
                                       UnicodeString& appendTo,
                                       FieldPositionIterator* posIter,
-                                      UErrorCode& status) const;
+                                      UErrorCode& status) const override;
         virtual void parseObject(const UnicodeString&,
                                  Formattable&,
-                                 ParsePosition&) const;
+                                 ParsePosition&) const override;
     };
 
     friend class MessageFormatAdapter; // getFormatTypeList() access
diff --git a/icu4c/source/i18n/unicode/numsys.h b/icu4c/source/i18n/unicode/numsys.h
index 1646729777f..4f839d0f354 100644
--- a/icu4c/source/i18n/unicode/numsys.h
+++ b/icu4c/source/i18n/unicode/numsys.h
@@ -192,7 +192,7 @@ public:
      *
      * @stable ICU 4.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 
 private:
diff --git a/icu4c/source/i18n/unicode/plurfmt.h b/icu4c/source/i18n/unicode/plurfmt.h
index bd2fbfc9fc8..bcabf3950b1 100644
--- a/icu4c/source/i18n/unicode/plurfmt.h
+++ b/icu4c/source/i18n/unicode/plurfmt.h
@@ -434,7 +434,7 @@ public:
       * @return         true if other is semantically equal to this.
       * @stable ICU 4.0
       */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
     /**
      * Return true if another object is semantically unequal to this one.
@@ -450,7 +450,7 @@ public:
      * result and should delete it when done.
      * @stable ICU 4.0
      */
-    virtual PluralFormat* clone() const;
+    virtual PluralFormat* clone() const override;
 
    /**
     * Formats a plural message for a number taken from a Formattable object.
@@ -469,7 +469,7 @@ public:
    UnicodeString& format(const Formattable& obj,
                          UnicodeString& appendTo,
                          FieldPosition& pos,
-                         UErrorCode& status) const;
+                         UErrorCode& status) const override;
 
    /**
     * Returns the pattern from applyPattern() or constructor().
@@ -505,7 +505,7 @@ public:
     */
    virtual void parseObject(const UnicodeString& source,
                             Formattable& result,
-                            ParsePosition& parse_pos) const;
+                            ParsePosition& parse_pos) const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -520,7 +520,7 @@ public:
      *
      * @stable ICU 4.0
      */
-     virtual UClassID getDynamicClassID() const;
+     virtual UClassID getDynamicClassID() const override;
 
 private:
      /**
@@ -548,7 +548,7 @@ private:
 
         virtual ~PluralSelectorAdapter();
 
-        virtual UnicodeString select(void *context, double number, UErrorCode& /*ec*/) const;
+        virtual UnicodeString select(void *context, double number, UErrorCode& /*ec*/) const override;
 
         void reset();
 
diff --git a/icu4c/source/i18n/unicode/plurrule.h b/icu4c/source/i18n/unicode/plurrule.h
index 1cc4225e79d..344bf1692b1 100644
--- a/icu4c/source/i18n/unicode/plurrule.h
+++ b/icu4c/source/i18n/unicode/plurrule.h
@@ -562,7 +562,7 @@ public:
      *
      * @stable ICU 4.0
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 
 private:
diff --git a/icu4c/source/i18n/unicode/rbnf.h b/icu4c/source/i18n/unicode/rbnf.h
index 3ea5c788ce3..0a5a378e13b 100644
--- a/icu4c/source/i18n/unicode/rbnf.h
+++ b/icu4c/source/i18n/unicode/rbnf.h
@@ -698,7 +698,7 @@ public:
    * @return  A copy of the object.
    * @stable ICU 2.6
    */
-  virtual RuleBasedNumberFormat* clone() const;
+  virtual RuleBasedNumberFormat* clone() const override;
 
   /**
    * Return true if the given Format objects are semantically equal.
@@ -707,7 +707,7 @@ public:
    * @return        true if the given Format objects are semantically equal.
    * @stable ICU 2.6
    */
-  virtual bool operator==(const Format& other) const;
+  virtual bool operator==(const Format& other) const override;
 
 //-----------------------------------------------------------------------
 // public API functions
@@ -793,7 +793,7 @@ public:
    */
   virtual UnicodeString& format(int32_t number,
                                 UnicodeString& toAppendTo,
-                                FieldPosition& pos) const;
+                                FieldPosition& pos) const override;
 
   /**
    * Formats the specified 64-bit number using the default ruleset.
@@ -805,7 +805,7 @@ public:
    */
   virtual UnicodeString& format(int64_t number,
                                 UnicodeString& toAppendTo,
-                                FieldPosition& pos) const;
+                                FieldPosition& pos) const override;
   /**
    * Formats the specified number using the default ruleset.
    * @param number The number to format.
@@ -816,7 +816,7 @@ public:
    */
   virtual UnicodeString& format(double number,
                                 UnicodeString& toAppendTo,
-                                FieldPosition& pos) const;
+                                FieldPosition& pos) const override;
 
   /**
    * Formats the specified number using the named ruleset.
@@ -888,7 +888,7 @@ protected:
     virtual UnicodeString& format(const number::impl::DecimalQuantity &number,
                                   UnicodeString& appendTo,
                                   FieldPosition& pos,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
 public:
 
   using NumberFormat::parse;
@@ -909,7 +909,7 @@ public:
    */
   virtual void parse(const UnicodeString& text,
                      Formattable& result,
-                     ParsePosition& parsePosition) const;
+                     ParsePosition& parsePosition) const override;
 
 #if !UCONFIG_NO_COLLATION
 
@@ -946,7 +946,7 @@ public:
    * @see RuleBasedCollator
    * @stable ICU 2.0
    */
-  virtual void setLenient(UBool enabled);
+  virtual void setLenient(UBool enabled) override;
 
   /**
    * Returns true if lenient-parse mode is turned on.  Lenient parsing is off
@@ -955,7 +955,7 @@ public:
    * @see #setLenient
    * @stable ICU 2.0
    */
-  virtual inline UBool isLenient(void) const;
+  virtual inline UBool isLenient(void) const override;
 
 #endif
 
@@ -987,21 +987,21 @@ public:
    *               updated with any new status from the function. 
    * @stable ICU 53
    */
-  virtual void setContext(UDisplayContext value, UErrorCode& status);
+  virtual void setContext(UDisplayContext value, UErrorCode& status) override;
 
     /**
      * Get the rounding mode.
      * @return A rounding mode
      * @stable ICU 60
      */
-    virtual ERoundingMode getRoundingMode(void) const;
+    virtual ERoundingMode getRoundingMode(void) const override;
 
     /**
      * Set the rounding mode.
      * @param roundingMode A rounding mode
      * @stable ICU 60
      */
-    virtual void setRoundingMode(ERoundingMode roundingMode);
+    virtual void setRoundingMode(ERoundingMode roundingMode) override;
 
 public:
     /**
@@ -1016,7 +1016,7 @@ public:
      *
      * @stable ICU 2.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Sets the decimal format symbols, which is generally not changed
diff --git a/icu4c/source/i18n/unicode/rbtz.h b/icu4c/source/i18n/unicode/rbtz.h
index 082e21df95b..17f72ed67c1 100644
--- a/icu4c/source/i18n/unicode/rbtz.h
+++ b/icu4c/source/i18n/unicode/rbtz.h
@@ -76,7 +76,7 @@ public:
       *semantically equal.
      * @stable ICU 3.8
      */
-    virtual bool operator==(const TimeZone& that) const;
+    virtual bool operator==(const TimeZone& that) const override;
 
     /**
      * Return true if the given <code>TimeZone</code> objects are
@@ -123,7 +123,7 @@ public:
      * @return   A new copy of this TimeZone object.
      * @stable ICU 3.8
      */
-    virtual RuleBasedTimeZone* clone() const;
+    virtual RuleBasedTimeZone* clone() const override;
 
     /**
      * Returns the TimeZone's adjusted GMT offset (i.e., the number of milliseconds to add
@@ -150,7 +150,7 @@ public:
      * @stable ICU 3.8
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
-                              uint8_t dayOfWeek, int32_t millis, UErrorCode& status) const;
+                              uint8_t dayOfWeek, int32_t millis, UErrorCode& status) const override;
 
     /**
      * Gets the time zone offset, for current date, modified in case of
@@ -173,7 +173,7 @@ public:
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
                            uint8_t dayOfWeek, int32_t millis,
-                           int32_t monthLength, UErrorCode& status) const;
+                           int32_t monthLength, UErrorCode& status) const override;
 
     /**
      * Returns the time zone raw and GMT offset for the given moment
@@ -198,7 +198,7 @@ public:
      * @stable ICU 3.8
      */
     virtual void getOffset(UDate date, UBool local, int32_t& rawOffset,
-                           int32_t& dstOffset, UErrorCode& ec) const;
+                           int32_t& dstOffset, UErrorCode& ec) const override;
 
     /**
      * Sets the TimeZone's raw GMT offset (i.e., the number of milliseconds to add
@@ -207,7 +207,7 @@ public:
      * @param offsetMillis  The new raw GMT offset for this time zone.
      * @stable ICU 3.8
      */
-    virtual void setRawOffset(int32_t offsetMillis);
+    virtual void setRawOffset(int32_t offsetMillis) override;
 
     /**
      * Returns the TimeZone's raw GMT offset (i.e., the number of milliseconds to add
@@ -216,7 +216,7 @@ public:
      * @return   The TimeZone's raw GMT offset.
      * @stable ICU 3.8
      */
-    virtual int32_t getRawOffset(void) const;
+    virtual int32_t getRawOffset(void) const override;
 
     /**
      * Queries if this time zone uses daylight savings time.
@@ -224,7 +224,7 @@ public:
      * false, otherwise.
      * @stable ICU 3.8
      */
-    virtual UBool useDaylightTime(void) const;
+    virtual UBool useDaylightTime(void) const override;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
@@ -240,7 +240,7 @@ public:
      * false, otherwise.
      * @deprecated ICU 2.4. Use Calendar::inDaylightTime() instead.
      */
-    virtual UBool inDaylightTime(UDate date, UErrorCode& status) const;
+    virtual UBool inDaylightTime(UDate date, UErrorCode& status) const override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -251,7 +251,7 @@ public:
      * with the possible exception of the ID
      * @stable ICU 3.8
      */
-    virtual UBool hasSameRules(const TimeZone& other) const;
+    virtual UBool hasSameRules(const TimeZone& other) const override;
 
     /**
      * Gets the first time zone transition after the base time.
@@ -261,7 +261,7 @@ public:
      * @return  true if the transition is found.
      * @stable ICU 3.8
      */
-    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * Gets the most recent time zone transition before the base time.
@@ -271,7 +271,7 @@ public:
      * @return  true if the transition is found.
      * @stable ICU 3.8
      */
-    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * Returns the number of <code>TimeZoneRule</code>s which represents time transitions,
@@ -281,7 +281,7 @@ public:
      * @return The number of <code>TimeZoneRule</code>s representing time transitions.
      * @stable ICU 3.8
      */
-    virtual int32_t countTransitionRules(UErrorCode& status) const;
+    virtual int32_t countTransitionRules(UErrorCode& status) const override;
 
     /**
      * Gets the <code>InitialTimeZoneRule</code> and the set of <code>TimeZoneRule</code>
@@ -300,7 +300,7 @@ public:
      * @stable ICU 3.8
      */
     virtual void getTimeZoneRules(const InitialTimeZoneRule*& initial,
-        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const;
+        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const override;
 
 #ifndef U_FORCE_HIDE_DRAFT_API
     /**
@@ -310,7 +310,7 @@ public:
     virtual void getOffsetFromLocal(
         UDate date, UTimeZoneLocalOption nonExistingTimeOpt,
         UTimeZoneLocalOption duplicatedTimeOpt,
-        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const;
+        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const override;
 #endif /* U_FORCE_HIDE_DRAFT_API */
 
 private:
@@ -360,7 +360,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/unicode/regex.h b/icu4c/source/i18n/unicode/regex.h
index fc0d380cda0..f8e32922f4e 100644
--- a/icu4c/source/i18n/unicode/regex.h
+++ b/icu4c/source/i18n/unicode/regex.h
@@ -569,7 +569,7 @@ public:
      *
      * @stable ICU 2.4
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -1746,7 +1746,7 @@ public:
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     // Constructors and other object boilerplate are private.
diff --git a/icu4c/source/i18n/unicode/scientificnumberformatter.h b/icu4c/source/i18n/unicode/scientificnumberformatter.h
index 7f860d4f592..aa7e80543e4 100644
--- a/icu4c/source/i18n/unicode/scientificnumberformatter.h
+++ b/icu4c/source/i18n/unicode/scientificnumberformatter.h
@@ -159,14 +159,14 @@ public:
 
     class U_I18N_API SuperscriptStyle : public Style {
     public:
-        virtual SuperscriptStyle *clone() const;
+        virtual SuperscriptStyle *clone() const override;
     protected:
         virtual UnicodeString &format(
                 const UnicodeString &original,
                 FieldPositionIterator &fpi,
                 const UnicodeString &preExponent,
                 UnicodeString &appendTo,
-                UErrorCode &status) const;
+                UErrorCode &status) const override;
     };
 
     class U_I18N_API MarkupStyle : public Style {
@@ -177,14 +177,14 @@ public:
                 : Style(),
                   fBeginMarkup(beginMarkup),
                   fEndMarkup(endMarkup) { }
-        virtual MarkupStyle *clone() const;
+        virtual MarkupStyle *clone() const override;
     protected:
         virtual UnicodeString &format(
                 const UnicodeString &original,
                 FieldPositionIterator &fpi,
                 const UnicodeString &preExponent,
                 UnicodeString &appendTo,
-                UErrorCode &status) const;
+                UErrorCode &status) const override;
     private:
         UnicodeString fBeginMarkup;
         UnicodeString fEndMarkup;
diff --git a/icu4c/source/i18n/unicode/selfmt.h b/icu4c/source/i18n/unicode/selfmt.h
index 10c2ddff9f8..bdce00e593e 100644
--- a/icu4c/source/i18n/unicode/selfmt.h
+++ b/icu4c/source/i18n/unicode/selfmt.h
@@ -259,7 +259,7 @@ public:
      * @return         true if other is semantically equal to this.
      * @stable ICU 4.4
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
     /**
      * Return true if another object is semantically unequal to this one.
@@ -275,7 +275,7 @@ public:
      * result and should delete it when done.
      * @stable ICU 4.4
      */
-    virtual SelectFormat* clone() const;
+    virtual SelectFormat* clone() const override;
 
     /**
      * Format an object to produce a string.
@@ -295,7 +295,7 @@ public:
     UnicodeString& format(const Formattable& obj,
                          UnicodeString& appendTo,
                          FieldPosition& pos,
-                         UErrorCode& status) const;
+                         UErrorCode& status) const override;
 
     /**
      * Returns the pattern from applyPattern() or constructor.
@@ -331,7 +331,7 @@ public:
      */
     virtual void parseObject(const UnicodeString& source,
                             Formattable& result,
-                            ParsePosition& parse_pos) const;
+                            ParsePosition& parse_pos) const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -343,7 +343,7 @@ public:
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      * @stable ICU 4.4
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 private:
     friend class MessageFormat;
diff --git a/icu4c/source/i18n/unicode/simpletz.h b/icu4c/source/i18n/unicode/simpletz.h
index 8ead876a0ec..f5c155de466 100644
--- a/icu4c/source/i18n/unicode/simpletz.h
+++ b/icu4c/source/i18n/unicode/simpletz.h
@@ -111,7 +111,7 @@ public:
      *              otherwise.
      * @stable ICU 2.0
      */
-    virtual bool operator==(const TimeZone& that) const;
+    virtual bool operator==(const TimeZone& that) const override;
 
     /**
      * Constructs a SimpleTimeZone with the given raw GMT offset and time zone ID,
@@ -571,7 +571,7 @@ public:
      * @stable ICU 2.0
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
-                              uint8_t dayOfWeek, int32_t millis, UErrorCode& status) const;
+                              uint8_t dayOfWeek, int32_t millis, UErrorCode& status) const override;
 
     /**
      * Gets the time zone offset, for current date, modified in case of
@@ -590,7 +590,7 @@ public:
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
                            uint8_t dayOfWeek, int32_t milliseconds,
-                           int32_t monthLength, UErrorCode& status) const;
+                           int32_t monthLength, UErrorCode& status) const override;
     /**
      * Gets the time zone offset, for current date, modified in case of
      * daylight savings. This is the offset to add *to* UTC to get local time.
@@ -618,7 +618,7 @@ public:
      * @stable ICU 2.8
      */
     virtual void getOffset(UDate date, UBool local, int32_t& rawOffset,
-                           int32_t& dstOffset, UErrorCode& ec) const;
+                           int32_t& dstOffset, UErrorCode& ec) const override;
 
 #ifndef U_FORCE_HIDE_DRAFT_API
     /**
@@ -628,7 +628,7 @@ public:
     virtual void getOffsetFromLocal(
         UDate date, UTimeZoneLocalOption nonExistingTimeOpt,
         UTimeZoneLocalOption duplicatedTimeOpt,
-        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const;
+        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const override;
 #endif /* U_FORCE_HIDE_DRAFT_API */
 
     /**
@@ -638,7 +638,7 @@ public:
      * @return   The TimeZone's raw GMT offset.
      * @stable ICU 2.0
      */
-    virtual int32_t getRawOffset(void) const;
+    virtual int32_t getRawOffset(void) const override;
 
     /**
      * Sets the TimeZone's raw GMT offset (i.e., the number of milliseconds to add
@@ -647,7 +647,7 @@ public:
      * @param offsetMillis  The new raw GMT offset for this time zone.
      * @stable ICU 2.0
      */
-    virtual void setRawOffset(int32_t offsetMillis);
+    virtual void setRawOffset(int32_t offsetMillis) override;
 
     /**
      * Sets the amount of time in ms that the clock is advanced during DST.
@@ -668,7 +668,7 @@ public:
      * but not 0.
      * @stable ICU 2.0
      */
-    virtual int32_t getDSTSavings(void) const;
+    virtual int32_t getDSTSavings(void) const override;
 
     /**
      * Queries if this TimeZone uses Daylight Savings Time.
@@ -676,7 +676,7 @@ public:
      * @return   True if this TimeZone uses Daylight Savings Time; false otherwise.
      * @stable ICU 2.0
      */
-    virtual UBool useDaylightTime(void) const;
+    virtual UBool useDaylightTime(void) const override;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
@@ -693,7 +693,7 @@ public:
      * false otherwise.
      * @deprecated ICU 2.4. Use Calendar::inDaylightTime() instead.
      */
-    virtual UBool inDaylightTime(UDate date, UErrorCode& status) const;
+    virtual UBool inDaylightTime(UDate date, UErrorCode& status) const override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -702,7 +702,7 @@ public:
      * @return true if the given zone has the same rules and offset as this one
      * @stable ICU 2.0
      */
-    UBool hasSameRules(const TimeZone& other) const;
+    UBool hasSameRules(const TimeZone& other) const override;
 
     /**
      * Clones TimeZone objects polymorphically. Clients are responsible for deleting
@@ -711,7 +711,7 @@ public:
      * @return   A new copy of this TimeZone object.
      * @stable ICU 2.0
      */
-    virtual SimpleTimeZone* clone() const;
+    virtual SimpleTimeZone* clone() const override;
 
     /**
      * Gets the first time zone transition after the base time.
@@ -721,7 +721,7 @@ public:
      * @return  true if the transition is found.
      * @stable ICU 3.8
      */
-    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * Gets the most recent time zone transition before the base time.
@@ -731,7 +731,7 @@ public:
      * @return  true if the transition is found.
      * @stable ICU 3.8
      */
-    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * Returns the number of <code>TimeZoneRule</code>s which represents time transitions,
@@ -741,7 +741,7 @@ public:
      * @return The number of <code>TimeZoneRule</code>s representing time transitions.
      * @stable ICU 3.8
      */
-    virtual int32_t countTransitionRules(UErrorCode& status) const;
+    virtual int32_t countTransitionRules(UErrorCode& status) const override;
 
     /**
      * Gets the <code>InitialTimeZoneRule</code> and the set of <code>TimeZoneRule</code>
@@ -760,7 +760,7 @@ public:
      * @stable ICU 3.8
      */
     virtual void getTimeZoneRules(const InitialTimeZoneRule*& initial,
-        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const;
+        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const override;
 
 
 public:
@@ -775,7 +775,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to a return
diff --git a/icu4c/source/i18n/unicode/smpdtfmt.h b/icu4c/source/i18n/unicode/smpdtfmt.h
index a2269d637b7..733a1cff13b 100644
--- a/icu4c/source/i18n/unicode/smpdtfmt.h
+++ b/icu4c/source/i18n/unicode/smpdtfmt.h
@@ -867,7 +867,7 @@ public:
      * @return    A copy of the object.
      * @stable ICU 2.0
      */
-    virtual SimpleDateFormat* clone() const;
+    virtual SimpleDateFormat* clone() const override;
 
     /**
      * Return true if the given Format objects are semantically equal. Objects
@@ -876,7 +876,7 @@ public:
      * @return         true if the given Format objects are semantically equal.
      * @stable ICU 2.0
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
 
     using DateFormat::format;
@@ -899,7 +899,7 @@ public:
      */
     virtual UnicodeString& format(  Calendar& cal,
                                     UnicodeString& appendTo,
-                                    FieldPosition& pos) const;
+                                    FieldPosition& pos) const override;
 
     /**
      * Format a date or time, which is the standard millis since 24:00 GMT, Jan
@@ -922,7 +922,7 @@ public:
     virtual UnicodeString& format(  Calendar& cal,
                                     UnicodeString& appendTo,
                                     FieldPositionIterator* posIter,
-                                    UErrorCode& status) const;
+                                    UErrorCode& status) const override;
 
     using DateFormat::parse;
 
@@ -954,7 +954,7 @@ public:
      */
     virtual void parse( const UnicodeString& text,
                         Calendar& cal,
-                        ParsePosition& pos) const;
+                        ParsePosition& pos) const override;
 
 
     /**
@@ -1097,7 +1097,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Set the calendar to be used by this date format. Initially, the default
@@ -1108,7 +1108,7 @@ public:
      * @param calendarToAdopt    Calendar object to be adopted.
      * @stable ICU 2.0
      */
-    virtual void adoptCalendar(Calendar* calendarToAdopt);
+    virtual void adoptCalendar(Calendar* calendarToAdopt) override;
 
     /* Cannot use #ifndef U_HIDE_INTERNAL_API for the following methods since they are virtual */
     /**
@@ -1144,7 +1144,7 @@ public:
      *               updated with any new status from the function.
      * @stable ICU 53
      */
-    virtual void setContext(UDisplayContext value, UErrorCode& status);
+    virtual void setContext(UDisplayContext value, UErrorCode& status) override;
 
     /**
      * Overrides base class method and
@@ -1153,7 +1153,7 @@ public:
      * @param formatToAdopt the NumbeferFormat used
      * @stable ICU 54
      */
-    void adoptNumberFormat(NumberFormat *formatToAdopt);
+    void adoptNumberFormat(NumberFormat *formatToAdopt) override;
 
     /**
      * Allow the user to set the NumberFormat for several fields
diff --git a/icu4c/source/i18n/unicode/sortkey.h b/icu4c/source/i18n/unicode/sortkey.h
index 8ab8459e5c0..b1231f18b78 100644
--- a/icu4c/source/i18n/unicode/sortkey.h
+++ b/icu4c/source/i18n/unicode/sortkey.h
@@ -237,7 +237,7 @@ public:
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/i18n/unicode/stsearch.h b/icu4c/source/i18n/unicode/stsearch.h
index 69b75db605a..af1b2d4eabc 100644
--- a/icu4c/source/i18n/unicode/stsearch.h
+++ b/icu4c/source/i18n/unicode/stsearch.h
@@ -297,7 +297,7 @@ public:
      *         while looking for the same pattern.
      * @stable ICU 2.0
      */
-    virtual bool operator==(const SearchIterator &that) const;
+    virtual bool operator==(const SearchIterator &that) const override;
 
     // public get and set methods ----------------------------------------
 
@@ -314,7 +314,7 @@ public:
      * @param status for errors if it occurs
      * @stable ICU 2.0
      */
-    virtual void setOffset(int32_t position, UErrorCode &status);
+    virtual void setOffset(int32_t position, UErrorCode &status) override;
 
     /**
      * Return the current index in the text being searched.
@@ -324,7 +324,7 @@ public:
      * @return current index in the text being searched.
      * @stable ICU 2.0
      */
-    virtual int32_t getOffset(void) const;
+    virtual int32_t getOffset(void) const override;
 
     /**
      * Set the target text to be searched.
@@ -337,7 +337,7 @@ public:
      *        U_ILLEGAL_ARGUMENT_ERROR is returned.
      * @stable ICU 2.0
      */
-    virtual void setText(const UnicodeString &text, UErrorCode &status);
+    virtual void setText(const UnicodeString &text, UErrorCode &status) override;
     
     /**
      * Set the target text to be searched.
@@ -353,7 +353,7 @@ public:
      *        U_ILLEGAL_ARGUMENT_ERROR is returned.
      * @stable ICU 2.0
      */
-    virtual void setText(CharacterIterator &text, UErrorCode &status);
+    virtual void setText(CharacterIterator &text, UErrorCode &status) override;
 
     /**
      * Gets the collator used for the language rules.
@@ -403,7 +403,7 @@ public:
      * search will begin at the end of the text string.
      * @stable ICU 2.0
      */
-    virtual void reset();
+    virtual void reset() override;
 
     /**
      * Returns a copy of StringSearch with the same behavior, and 
@@ -413,14 +413,14 @@ public:
      * @return cloned object
      * @stable ICU 2.0
      */
-    virtual StringSearch * safeClone() const;
+    virtual StringSearch * safeClone() const override;
     
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      *
      * @stable ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -455,7 +455,7 @@ protected:
      *         USEARCH_DONE if no match was found.
      * @stable ICU 2.0
      */
-    virtual int32_t handleNext(int32_t position, UErrorCode &status);
+    virtual int32_t handleNext(int32_t position, UErrorCode &status) override;
 
     /**
      * Search backward for matching text, starting at a given location.
@@ -479,7 +479,7 @@ protected:
      *         USEARCH_DONE if no match was found.
      * @stable ICU 2.0
      */
-    virtual int32_t handlePrev(int32_t position, UErrorCode &status);
+    virtual int32_t handlePrev(int32_t position, UErrorCode &status) override;
     
 private :
     StringSearch(); // default constructor not implemented
diff --git a/icu4c/source/i18n/unicode/tblcoll.h b/icu4c/source/i18n/unicode/tblcoll.h
index 8e1d0822ae8..2de1af85392 100644
--- a/icu4c/source/i18n/unicode/tblcoll.h
+++ b/icu4c/source/i18n/unicode/tblcoll.h
@@ -223,14 +223,14 @@ public:
      * @return true if arguments is the same as this object.
      * @stable ICU 2.0
      */
-    virtual bool operator==(const Collator& other) const;
+    virtual bool operator==(const Collator& other) const override;
 
     /**
      * Makes a copy of this object.
      * @return a copy of this object, owned by the caller
      * @stable ICU 2.0
      */
-    virtual RuleBasedCollator* clone() const;
+    virtual RuleBasedCollator* clone() const override;
 
     /**
      * Creates a collation element iterator for the source string. The caller of
@@ -274,7 +274,7 @@ public:
     **/
     virtual UCollationResult compare(const UnicodeString& source,
                                      const UnicodeString& target,
-                                     UErrorCode &status) const;
+                                     UErrorCode &status) const override;
 
     /**
     * Does the same thing as compare but limits the comparison to a specified
@@ -292,7 +292,7 @@ public:
     virtual UCollationResult compare(const UnicodeString& source,
                                      const UnicodeString& target,
                                      int32_t length,
-                                     UErrorCode &status) const;
+                                     UErrorCode &status) const override;
 
     /**
     * The comparison function compares the character data stored in two
@@ -312,7 +312,7 @@ public:
     */
     virtual UCollationResult compare(const char16_t* source, int32_t sourceLength,
                                      const char16_t* target, int32_t targetLength,
-                                     UErrorCode &status) const;
+                                     UErrorCode &status) const override;
 
     /**
      * Compares two strings using the Collator.
@@ -327,7 +327,7 @@ public:
      */
     virtual UCollationResult compare(UCharIterator &sIter,
                                      UCharIterator &tIter,
-                                     UErrorCode &status) const;
+                                     UErrorCode &status) const override;
 
     /**
      * Compares two UTF-8 strings using the Collator.
@@ -344,7 +344,7 @@ public:
      */
     virtual UCollationResult compareUTF8(const StringPiece &source,
                                          const StringPiece &target,
-                                         UErrorCode &status) const;
+                                         UErrorCode &status) const override;
 
     /**
      * Transforms the string into a series of characters
@@ -362,7 +362,7 @@ public:
      */
     virtual CollationKey& getCollationKey(const UnicodeString& source,
                                           CollationKey& key,
-                                          UErrorCode& status) const;
+                                          UErrorCode& status) const override;
 
     /**
      * Transforms a specified region of the string into a series of characters
@@ -382,14 +382,14 @@ public:
     virtual CollationKey& getCollationKey(const char16_t *source,
                                           int32_t sourceLength,
                                           CollationKey& key,
-                                          UErrorCode& status) const;
+                                          UErrorCode& status) const override;
 
     /**
      * Generates the hash code for the rule-based collation object.
      * @return the hash code.
      * @stable ICU 2.0
      */
-    virtual int32_t hashCode() const;
+    virtual int32_t hashCode() const override;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
@@ -402,7 +402,7 @@ public:
     *         was instantiated from rules, locale is empty.
     * @deprecated ICU 2.8 likely to change in ICU 3.0, based on feedback
     */
-    virtual Locale getLocale(ULocDataLocaleType type, UErrorCode& status) const;
+    virtual Locale getLocale(ULocDataLocaleType type, UErrorCode& status) const override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -417,7 +417,7 @@ public:
      * @param info the version # information, the result will be filled in
      * @stable ICU 2.0
      */
-    virtual void getVersion(UVersionInfo info) const;
+    virtual void getVersion(UVersionInfo info) const override;
 
 #ifndef U_HIDE_DEPRECATED_API
     /**
@@ -449,7 +449,7 @@ public:
      *         IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     /**
      * Returns the class ID for this class. This is useful only for comparing to
@@ -511,7 +511,7 @@ public:
      * @stable ICU 2.2
      */
     virtual void setAttribute(UColAttribute attr, UColAttributeValue value,
-                              UErrorCode &status);
+                              UErrorCode &status) override;
 
     /**
      * Universal attribute getter.
@@ -521,7 +521,7 @@ public:
      * @stable ICU 2.2
      */
     virtual UColAttributeValue getAttribute(UColAttribute attr,
-                                            UErrorCode &status) const;
+                                            UErrorCode &status) const override;
 
     /**
      * Sets the variable top to the top of the specified reordering group.
@@ -539,7 +539,7 @@ public:
      * @see getMaxVariable
      * @stable ICU 53
      */
-    virtual Collator &setMaxVariable(UColReorderCode group, UErrorCode &errorCode);
+    virtual Collator &setMaxVariable(UColReorderCode group, UErrorCode &errorCode) override;
 
     /**
      * Returns the maximum reordering group whose characters are affected by UCOL_ALTERNATE_HANDLING.
@@ -547,7 +547,7 @@ public:
      * @see setMaxVariable
      * @stable ICU 53
      */
-    virtual UColReorderCode getMaxVariable() const;
+    virtual UColReorderCode getMaxVariable() const override;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
@@ -566,7 +566,7 @@ public:
      * @return variable top primary weight
      * @deprecated ICU 53 Call setMaxVariable() instead.
      */
-    virtual uint32_t setVariableTop(const char16_t *varTop, int32_t len, UErrorCode &status);
+    virtual uint32_t setVariableTop(const char16_t *varTop, int32_t len, UErrorCode &status) override;
 
     /**
      * Sets the variable top to the primary weight of the specified string.
@@ -583,7 +583,7 @@ public:
      * @return variable top primary weight
      * @deprecated ICU 53 Call setMaxVariable() instead.
      */
-    virtual uint32_t setVariableTop(const UnicodeString &varTop, UErrorCode &status);
+    virtual uint32_t setVariableTop(const UnicodeString &varTop, UErrorCode &status) override;
 
     /**
      * Sets the variable top to the specified primary weight.
@@ -596,7 +596,7 @@ public:
      * @param status error code
      * @deprecated ICU 53 Call setMaxVariable() instead.
      */
-    virtual void setVariableTop(uint32_t varTop, UErrorCode &status);
+    virtual void setVariableTop(uint32_t varTop, UErrorCode &status) override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -606,7 +606,7 @@ public:
      * @see getMaxVariable
      * @stable ICU 2.0
      */
-    virtual uint32_t getVariableTop(UErrorCode &status) const;
+    virtual uint32_t getVariableTop(UErrorCode &status) const override;
 
     /**
      * Get a UnicodeSet that contains all the characters and sequences tailored in
@@ -617,7 +617,7 @@ public:
      *         in the root collator. The object must be disposed of by using delete
      * @stable ICU 2.4
      */
-    virtual UnicodeSet *getTailoredSet(UErrorCode &status) const;
+    virtual UnicodeSet *getTailoredSet(UErrorCode &status) const override;
 
     /**
      * Get the sort key as an array of bytes from a UnicodeString.
@@ -634,7 +634,7 @@ public:
      * @stable ICU 2.0
      */
     virtual int32_t getSortKey(const UnicodeString& source, uint8_t *result,
-                               int32_t resultLength) const;
+                               int32_t resultLength) const override;
 
     /**
      * Get the sort key as an array of bytes from a char16_t buffer.
@@ -653,7 +653,7 @@ public:
      * @stable ICU 2.2
      */
     virtual int32_t getSortKey(const char16_t *source, int32_t sourceLength,
-                               uint8_t *result, int32_t resultLength) const;
+                               uint8_t *result, int32_t resultLength) const override;
 
     /**
      * Retrieves the reordering codes for this collator.
@@ -670,7 +670,7 @@ public:
      */
      virtual int32_t getReorderCodes(int32_t *dest,
                                      int32_t destCapacity,
-                                     UErrorCode& status) const;
+                                     UErrorCode& status) const override;
 
     /**
      * Sets the ordering of scripts for this collator.
@@ -685,7 +685,7 @@ public:
      */
      virtual void setReorderCodes(const int32_t* reorderCodes,
                                   int32_t reorderCodesLength,
-                                  UErrorCode& status) ;
+                                  UErrorCode& status) override;
 
     /**
      * Implements ucol_strcollUTF8().
@@ -694,7 +694,7 @@ public:
     virtual UCollationResult internalCompareUTF8(
             const char *left, int32_t leftLength,
             const char *right, int32_t rightLength,
-            UErrorCode &errorCode) const;
+            UErrorCode &errorCode) const override;
 
     /** Get the short definition string for a collator. This internal API harvests the collator's
      *  locale and the attribute set and produces a string that can be used for opening
@@ -722,7 +722,7 @@ public:
     virtual int32_t internalGetShortDefinitionString(const char *locale,
                                                      char *buffer,
                                                      int32_t capacity,
-                                                     UErrorCode &status) const;
+                                                     UErrorCode &status) const override;
 
     /**
      * Implements ucol_nextSortKeyPart().
@@ -730,7 +730,7 @@ public:
      */
     virtual int32_t internalNextSortKeyPart(
             UCharIterator *iter, uint32_t state[2],
-            uint8_t *dest, int32_t count, UErrorCode &errorCode) const;
+            uint8_t *dest, int32_t count, UErrorCode &errorCode) const override;
 
     // Do not enclose the default constructor with #ifndef U_HIDE_INTERNAL_API
     /**
@@ -806,7 +806,7 @@ protected:
     * @param actualLocale the actual locale
     * @internal
     */
-    virtual void setLocales(const Locale& requestedLocale, const Locale& validLocale, const Locale& actualLocale);
+    virtual void setLocales(const Locale& requestedLocale, const Locale& validLocale, const Locale& actualLocale) override;
 
 private:
     friend class CollationElementIterator;
diff --git a/icu4c/source/i18n/unicode/tmunit.h b/icu4c/source/i18n/unicode/tmunit.h
index 2e949ddfbd5..24abb49f198 100644
--- a/icu4c/source/i18n/unicode/tmunit.h
+++ b/icu4c/source/i18n/unicode/tmunit.h
@@ -73,7 +73,7 @@ public:
      * Override clone.
      * @stable ICU 4.2
      */
-    virtual TimeUnit* clone() const;
+    virtual TimeUnit* clone() const override;
 
     /**
      * Copy operator.
@@ -95,7 +95,7 @@ public:
      * different class IDs.
      * @stable ICU 4.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * Returns the class ID for this class. This is used to compare to
diff --git a/icu4c/source/i18n/unicode/tmutamt.h b/icu4c/source/i18n/unicode/tmutamt.h
index 4f7124f39d8..88e892fb0c2 100644
--- a/icu4c/source/i18n/unicode/tmutamt.h
+++ b/icu4c/source/i18n/unicode/tmutamt.h
@@ -87,7 +87,7 @@ public:
      * @return a polymorphic clone of this object. The result will have the same               class as returned by getDynamicClassID().
      * @stable ICU 4.2
      */
-    virtual TimeUnitAmount* clone() const;
+    virtual TimeUnitAmount* clone() const override;
 
     
     /**
@@ -140,7 +140,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 4.2
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
 
     /**
diff --git a/icu4c/source/i18n/unicode/tmutfmt.h b/icu4c/source/i18n/unicode/tmutfmt.h
index 24ba906b5ea..02e0563a010 100644
--- a/icu4c/source/i18n/unicode/tmutfmt.h
+++ b/icu4c/source/i18n/unicode/tmutfmt.h
@@ -126,7 +126,7 @@ public:
      * @return    A copy of the object.
      * @deprecated ICU 53
      */
-    virtual TimeUnitFormat* clone() const;
+    virtual TimeUnitFormat* clone() const override;
 
     /**
      * Assignment operator
@@ -158,7 +158,7 @@ public:
      */
     virtual void parseObject(const UnicodeString& source,
                              Formattable& result,
-                             ParsePosition& pos) const;
+                             ParsePosition& pos) const override;
 
     /**
      * Return the class ID for this class. This is useful only for comparing to
@@ -184,7 +184,7 @@ public:
      *                  other classes have different class IDs.
      * @deprecated ICU 53
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
 private:
     Hashtable*    fTimeUnitToCountToPatterns[TimeUnit::UTIMEUNIT_FIELD_COUNT];
diff --git a/icu4c/source/i18n/unicode/tzfmt.h b/icu4c/source/i18n/unicode/tzfmt.h
index d3cfda65227..6db8477026a 100644
--- a/icu4c/source/i18n/unicode/tzfmt.h
+++ b/icu4c/source/i18n/unicode/tzfmt.h
@@ -299,7 +299,7 @@ public:
      *                Objects of different subclasses are considered unequal.
      * @stable ICU 50
      */
-    virtual bool operator==(const Format& other) const;
+    virtual bool operator==(const Format& other) const override;
 
     /**
      * Clone this object polymorphically. The caller is responsible
@@ -307,7 +307,7 @@ public:
      * @return A copy of the object
      * @stable ICU 50
      */
-    virtual TimeZoneFormat* clone() const;
+    virtual TimeZoneFormat* clone() const override;
 
     /**
      * Creates an instance of <code>TimeZoneFormat</code> for the given locale.
@@ -636,7 +636,7 @@ public:
      * @stable ICU 50
      */
     virtual UnicodeString& format(const Formattable& obj, UnicodeString& appendTo,
-        FieldPosition& pos, UErrorCode& status) const;
+        FieldPosition& pos, UErrorCode& status) const override;
 
     /**
      * Parse a string to produce an object. This methods handles parsing of
@@ -650,7 +650,7 @@ public:
      *                 delete it when done.
      * @stable ICU 50
      */
-    virtual void parseObject(const UnicodeString& source, Formattable& result, ParsePosition& parse_pos) const;
+    virtual void parseObject(const UnicodeString& source, Formattable& result, ParsePosition& parse_pos) const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -662,7 +662,7 @@ public:
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      * @stable ICU 50
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
 protected:
     /**
diff --git a/icu4c/source/i18n/unicode/tzrule.h b/icu4c/source/i18n/unicode/tzrule.h
index 676b092309c..ab0b97b6fd2 100644
--- a/icu4c/source/i18n/unicode/tzrule.h
+++ b/icu4c/source/i18n/unicode/tzrule.h
@@ -231,7 +231,7 @@ public:
      * @return    A copy of the object.
      * @stable ICU 3.8
      */
-    virtual InitialTimeZoneRule* clone() const;
+    virtual InitialTimeZoneRule* clone() const override;
 
     /**
      * Assignment operator.
@@ -247,7 +247,7 @@ public:
      * @return  true if the given <code>TimeZoneRule</code> objects are semantically equal.
      * @stable ICU 3.8
      */
-    virtual bool operator==(const TimeZoneRule& that) const;
+    virtual bool operator==(const TimeZoneRule& that) const override;
 
     /**
      * Return true if the given <code>TimeZoneRule</code> objects are semantically unequal. Objects
@@ -256,7 +256,7 @@ public:
      * @return  true if the given <code>TimeZoneRule</code> objects are semantically unequal.
      * @stable ICU 3.8
      */
-    virtual bool operator!=(const TimeZoneRule& that) const;
+    virtual bool operator!=(const TimeZoneRule& that) const override;
 
     /**
      * Gets the time when this rule takes effect in the given year.
@@ -280,7 +280,7 @@ public:
      * @return  true if the other <code>TimeZoneRule</code> is equivalent to this one.
      * @stable ICU 3.8
      */
-    virtual UBool isEquivalentTo(const TimeZoneRule& that) const;
+    virtual UBool isEquivalentTo(const TimeZoneRule& that) const override;
 
     /**
      * Gets the very first time when this rule takes effect.
@@ -293,7 +293,7 @@ public:
      *          "result" is unchanged.
      * @stable ICU 3.8
      */
-    virtual UBool getFirstStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const;
+    virtual UBool getFirstStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const override;
 
     /**
      * Gets the final time when this rule takes effect.
@@ -306,7 +306,7 @@ public:
      *          "result" is unchanged.
      * @stable ICU 3.8
      */
-    virtual UBool getFinalStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const;
+    virtual UBool getFinalStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const override;
 
     /**
      * Gets the first time when this rule takes effect after the specified time.
@@ -323,7 +323,7 @@ public:
      * @stable ICU 3.8
      */
     virtual UBool getNextStart(UDate base, int32_t prevRawOffset, int32_t prevDSTSavings,
-        UBool inclusive, UDate& result) const;
+        UBool inclusive, UDate& result) const override;
 
     /**
      * Gets the most recent time when this rule takes effect before the specified time.
@@ -340,7 +340,7 @@ public:
      * @stable ICU 3.8
      */
     virtual UBool getPreviousStart(UDate base, int32_t prevRawOffset, int32_t prevDSTSavings,
-        UBool inclusive, UDate& result) const;
+        UBool inclusive, UDate& result) const override;
 
 public:
     /**
@@ -367,7 +367,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 /**
@@ -442,7 +442,7 @@ public:
      * @return    A copy of the object.
      * @stable ICU 3.8
      */
-    virtual AnnualTimeZoneRule* clone() const;
+    virtual AnnualTimeZoneRule* clone() const override;
 
     /**
      * Assignment operator.
@@ -458,7 +458,7 @@ public:
      * @return  true if the given <code>TimeZoneRule</code> objects are semantically equal.
      * @stable ICU 3.8
      */
-    virtual bool operator==(const TimeZoneRule& that) const;
+    virtual bool operator==(const TimeZoneRule& that) const override;
 
     /**
      * Return true if the given <code>TimeZoneRule</code> objects are semantically unequal. Objects
@@ -467,7 +467,7 @@ public:
      * @return  true if the given <code>TimeZoneRule</code> objects are semantically unequal.
      * @stable ICU 3.8
      */
-    virtual bool operator!=(const TimeZoneRule& that) const;
+    virtual bool operator!=(const TimeZoneRule& that) const override;
 
     /**
      * Gets the start date/time rule used by this rule.
@@ -515,7 +515,7 @@ public:
      * @return  true if the other <code>TimeZoneRule</code> is equivalent to this one.
      * @stable ICU 3.8
      */
-    virtual UBool isEquivalentTo(const TimeZoneRule& that) const;
+    virtual UBool isEquivalentTo(const TimeZoneRule& that) const override;
 
     /**
      * Gets the very first time when this rule takes effect.
@@ -528,7 +528,7 @@ public:
      *          "result" is unchanged.
      * @stable ICU 3.8
      */
-    virtual UBool getFirstStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const;
+    virtual UBool getFirstStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const override;
 
     /**
      * Gets the final time when this rule takes effect.
@@ -541,7 +541,7 @@ public:
      *          "result" is unchanged.
      * @stable ICU 3.8
      */
-    virtual UBool getFinalStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const;
+    virtual UBool getFinalStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const override;
 
     /**
      * Gets the first time when this rule takes effect after the specified time.
@@ -558,7 +558,7 @@ public:
      * @stable ICU 3.8
      */
     virtual UBool getNextStart(UDate base, int32_t prevRawOffset, int32_t prevDSTSavings,
-        UBool inclusive, UDate& result) const;
+        UBool inclusive, UDate& result) const override;
 
     /**
      * Gets the most recent time when this rule takes effect before the specified time.
@@ -575,7 +575,7 @@ public:
      * @stable ICU 3.8
      */
     virtual UBool getPreviousStart(UDate base, int32_t prevRawOffset, int32_t prevDSTSavings,
-        UBool inclusive, UDate& result) const;
+        UBool inclusive, UDate& result) const override;
 
 
 private:
@@ -608,7 +608,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 /**
@@ -658,7 +658,7 @@ public:
      * @return    A copy of the object.
      * @stable ICU 3.8
      */
-    virtual TimeArrayTimeZoneRule* clone() const;
+    virtual TimeArrayTimeZoneRule* clone() const override;
 
     /**
      * Assignment operator.
@@ -674,7 +674,7 @@ public:
      * @return  true if the given <code>TimeZoneRule</code> objects are semantically equal.
      * @stable ICU 3.8
      */
-    virtual bool operator==(const TimeZoneRule& that) const;
+    virtual bool operator==(const TimeZoneRule& that) const override;
 
     /**
      * Return true if the given <code>TimeZoneRule</code> objects are semantically unequal. Objects
@@ -683,7 +683,7 @@ public:
      * @return  true if the given <code>TimeZoneRule</code> objects are semantically unequal.
      * @stable ICU 3.8
      */
-    virtual bool operator!=(const TimeZoneRule& that) const;
+    virtual bool operator!=(const TimeZoneRule& that) const override;
 
     /**
      * Gets the time type of the start times used by this rule.  The return value
@@ -721,7 +721,7 @@ public:
      * @return  true if the other <code>TimeZoneRule</code> is equivalent to this one.
      * @stable ICU 3.8
      */
-    virtual UBool isEquivalentTo(const TimeZoneRule& that) const;
+    virtual UBool isEquivalentTo(const TimeZoneRule& that) const override;
 
     /**
      * Gets the very first time when this rule takes effect.
@@ -734,7 +734,7 @@ public:
      *          "result" is unchanged.
      * @stable ICU 3.8
      */
-    virtual UBool getFirstStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const;
+    virtual UBool getFirstStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const override;
 
     /**
      * Gets the final time when this rule takes effect.
@@ -747,7 +747,7 @@ public:
      *          "result" is unchanged.
      * @stable ICU 3.8
      */
-    virtual UBool getFinalStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const;
+    virtual UBool getFinalStart(int32_t prevRawOffset, int32_t prevDSTSavings, UDate& result) const override;
 
     /**
      * Gets the first time when this rule takes effect after the specified time.
@@ -764,7 +764,7 @@ public:
      * @stable ICU 3.8
      */
     virtual UBool getNextStart(UDate base, int32_t prevRawOffset, int32_t prevDSTSavings,
-        UBool inclusive, UDate& result) const;
+        UBool inclusive, UDate& result) const override;
 
     /**
      * Gets the most recent time when this rule takes effect before the specified time.
@@ -781,7 +781,7 @@ public:
      * @stable ICU 3.8
      */
     virtual UBool getPreviousStart(UDate base, int32_t prevRawOffset, int32_t prevDSTSavings,
-        UBool inclusive, UDate& result) const;
+        UBool inclusive, UDate& result) const override;
 
 
 private:
@@ -819,7 +819,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 
diff --git a/icu4c/source/i18n/unicode/tztrans.h b/icu4c/source/i18n/unicode/tztrans.h
index 03d96184653..5adbeb35e43 100644
--- a/icu4c/source/i18n/unicode/tztrans.h
+++ b/icu4c/source/i18n/unicode/tztrans.h
@@ -187,7 +187,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/unicode/vtzone.h b/icu4c/source/i18n/unicode/vtzone.h
index b72ab9ae5de..e7d2f515410 100644
--- a/icu4c/source/i18n/unicode/vtzone.h
+++ b/icu4c/source/i18n/unicode/vtzone.h
@@ -72,7 +72,7 @@ public:
       *semantically equal.
      * @stable ICU 3.8
      */
-    virtual bool operator==(const TimeZone& that) const;
+    virtual bool operator==(const TimeZone& that) const override;
 
     /**
      * Return true if the given <code>TimeZone</code> objects are
@@ -187,7 +187,7 @@ public:
      * @return   A new copy of this TimeZone object.
      * @stable ICU 3.8
      */
-    virtual VTimeZone* clone() const;
+    virtual VTimeZone* clone() const override;
 
     /**
      * Returns the TimeZone's adjusted GMT offset (i.e., the number of milliseconds to add
@@ -214,7 +214,7 @@ public:
      * @stable ICU 3.8
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
-                              uint8_t dayOfWeek, int32_t millis, UErrorCode& status) const;
+                              uint8_t dayOfWeek, int32_t millis, UErrorCode& status) const override;
 
     /**
      * Gets the time zone offset, for current date, modified in case of
@@ -237,7 +237,7 @@ public:
      */
     virtual int32_t getOffset(uint8_t era, int32_t year, int32_t month, int32_t day,
                            uint8_t dayOfWeek, int32_t millis,
-                           int32_t monthLength, UErrorCode& status) const;
+                           int32_t monthLength, UErrorCode& status) const override;
 
     /**
      * Returns the time zone raw and GMT offset for the given moment
@@ -262,7 +262,7 @@ public:
      * @stable ICU 3.8
      */
     virtual void getOffset(UDate date, UBool local, int32_t& rawOffset,
-                           int32_t& dstOffset, UErrorCode& ec) const;
+                           int32_t& dstOffset, UErrorCode& ec) const override;
 
 #ifndef U_FORCE_HIDE_DRAFT_API
     /**
@@ -272,7 +272,7 @@ public:
     virtual void getOffsetFromLocal(
         UDate date, UTimeZoneLocalOption nonExistingTimeOpt,
         UTimeZoneLocalOption duplicatedTimeOpt,
-        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const;
+        int32_t& rawOffset, int32_t& dstOffset, UErrorCode& status) const override;
 #endif /* U_FORCE_HIDE_DRAFT_API */
 
     /**
@@ -282,7 +282,7 @@ public:
      * @param offsetMillis  The new raw GMT offset for this time zone.
      * @stable ICU 3.8
      */
-    virtual void setRawOffset(int32_t offsetMillis);
+    virtual void setRawOffset(int32_t offsetMillis) override;
 
     /**
      * Returns the TimeZone's raw GMT offset (i.e., the number of milliseconds to add
@@ -291,7 +291,7 @@ public:
      * @return   The TimeZone's raw GMT offset.
      * @stable ICU 3.8
      */
-    virtual int32_t getRawOffset(void) const;
+    virtual int32_t getRawOffset(void) const override;
 
     /**
      * Queries if this time zone uses daylight savings time.
@@ -299,7 +299,7 @@ public:
      * false, otherwise.
      * @stable ICU 3.8
      */
-    virtual UBool useDaylightTime(void) const;
+    virtual UBool useDaylightTime(void) const override;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
@@ -315,7 +315,7 @@ public:
      * false, otherwise.
      * @deprecated ICU 2.4. Use Calendar::inDaylightTime() instead.
      */
-    virtual UBool inDaylightTime(UDate date, UErrorCode& status) const;
+    virtual UBool inDaylightTime(UDate date, UErrorCode& status) const override;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
@@ -326,7 +326,7 @@ public:
      * with the possible exception of the ID
      * @stable ICU 3.8
      */
-    virtual UBool hasSameRules(const TimeZone& other) const;
+    virtual UBool hasSameRules(const TimeZone& other) const override;
 
     /**
      * Gets the first time zone transition after the base time.
@@ -336,7 +336,7 @@ public:
      * @return  true if the transition is found.
      * @stable ICU 3.8
      */
-    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getNextTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * Gets the most recent time zone transition before the base time.
@@ -346,7 +346,7 @@ public:
      * @return  true if the transition is found.
      * @stable ICU 3.8
      */
-    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const;
+    virtual UBool getPreviousTransition(UDate base, UBool inclusive, TimeZoneTransition& result) const override;
 
     /**
      * Returns the number of <code>TimeZoneRule</code>s which represents time transitions,
@@ -356,7 +356,7 @@ public:
      * @return The number of <code>TimeZoneRule</code>s representing time transitions.
      * @stable ICU 3.8
      */
-    virtual int32_t countTransitionRules(UErrorCode& status) const;
+    virtual int32_t countTransitionRules(UErrorCode& status) const override;
 
     /**
      * Gets the <code>InitialTimeZoneRule</code> and the set of <code>TimeZoneRule</code>
@@ -375,7 +375,7 @@ public:
      * @stable ICU 3.8
      */
     virtual void getTimeZoneRules(const InitialTimeZoneRule*& initial,
-        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const;
+        const TimeZoneRule* trsrules[], int32_t& trscount, UErrorCode& status) const override;
 
 private:
     enum { DEFAULT_VTIMEZONE_LINES = 100 };
@@ -461,7 +461,7 @@ public:
      *                  other classes have different class IDs.
      * @stable ICU 3.8
      */
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/units_data.cpp b/icu4c/source/i18n/units_data.cpp
index 61f537479fa..d1d1fc5bc05 100644
--- a/icu4c/source/i18n/units_data.cpp
+++ b/icu4c/source/i18n/units_data.cpp
@@ -59,7 +59,7 @@ class ConversionRateDataSink : public ResourceSink {
      * @param noFallback Ignored.
      * @param status The standard ICU error code output parameter.
      */
-    void put(const char *source, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) {
+    void put(const char *source, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) override {
         if (U_FAILURE(status)) { return; }
         if (uprv_strcmp(source, "convertUnits") != 0) {
             // This is very strict, however it is the cheapest way to be sure
@@ -146,7 +146,7 @@ class UnitPreferencesSink : public ResourceSink {
      * @param status The standard ICU error code output parameter. Note: if an
      * error is returned, outPrefs and outMetadata may be inconsistent.
      */
-    void put(const char *key, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) {
+    void put(const char *key, ResourceValue &value, UBool /*noFallback*/, UErrorCode &status) override {
         if (U_FAILURE(status)) { return; }
         if (uprv_strcmp(key, "unitPreferenceData") != 0) {
             // This is very strict, however it is the cheapest way to be sure
diff --git a/icu4c/source/i18n/uspoof_impl.h b/icu4c/source/i18n/uspoof_impl.h
index 7002c945d1d..e75ae262bdd 100644
--- a/icu4c/source/i18n/uspoof_impl.h
+++ b/icu4c/source/i18n/uspoof_impl.h
@@ -93,7 +93,7 @@ public:
     static UChar32 ScanHex(const UChar *s, int32_t start, int32_t limit, UErrorCode &status);
 
     static UClassID U_EXPORT2 getStaticClassID(void);
-    virtual UClassID getDynamicClassID(void) const;
+    virtual UClassID getDynamicClassID(void) const override;
 
     //
     // Data Members
diff --git a/icu4c/source/i18n/utf16collationiterator.h b/icu4c/source/i18n/utf16collationiterator.h
index 5bb7ab1eebf..6db70511b0d 100644
--- a/icu4c/source/i18n/utf16collationiterator.h
+++ b/icu4c/source/i18n/utf16collationiterator.h
@@ -42,11 +42,11 @@ public:
 
     virtual ~UTF16CollationIterator();
 
-    virtual bool operator==(const CollationIterator &other) const;
+    virtual bool operator==(const CollationIterator &other) const override;
 
-    virtual void resetToOffset(int32_t newOffset);
+    virtual void resetToOffset(int32_t newOffset) override;
 
-    virtual int32_t getOffset() const;
+    virtual int32_t getOffset() const override;
 
     void setText(const UChar *s, const UChar *lim) {
         reset();
@@ -54,9 +54,9 @@ public:
         limit = lim;
     }
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
 
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
     // Copy constructor only for subclasses which set the pointers.
@@ -64,15 +64,15 @@ protected:
             : CollationIterator(other),
               start(NULL), pos(NULL), limit(NULL) {}
 
-    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode);
+    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode) override;
 
-    virtual UChar handleGetTrailSurrogate();
+    virtual UChar handleGetTrailSurrogate() override;
 
-    virtual UBool foundNULTerminator();
+    virtual UBool foundNULTerminator() override;
 
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
     // UTF-16 string pointers.
     // limit can be NULL for NUL-terminated strings.
@@ -95,24 +95,24 @@ public:
 
     virtual ~FCDUTF16CollationIterator();
 
-    virtual bool operator==(const CollationIterator &other) const;
+    virtual bool operator==(const CollationIterator &other) const override;
 
-    virtual void resetToOffset(int32_t newOffset);
+    virtual void resetToOffset(int32_t newOffset) override;
 
-    virtual int32_t getOffset() const;
+    virtual int32_t getOffset() const override;
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
 
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
-    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode);
+    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode) override;
 
-    virtual UBool foundNULTerminator();
+    virtual UBool foundNULTerminator() override;
 
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
 private:
     /**
diff --git a/icu4c/source/i18n/utf8collationiterator.h b/icu4c/source/i18n/utf8collationiterator.h
index 9059e72dcf9..09cfce4369f 100644
--- a/icu4c/source/i18n/utf8collationiterator.h
+++ b/icu4c/source/i18n/utf8collationiterator.h
@@ -40,13 +40,13 @@ public:
 
     virtual ~UTF8CollationIterator();
 
-    virtual void resetToOffset(int32_t newOffset);
+    virtual void resetToOffset(int32_t newOffset) override;
 
-    virtual int32_t getOffset() const;
+    virtual int32_t getOffset() const override;
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
 
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
     /**
@@ -59,15 +59,15 @@ protected:
      * Valid lead surrogates are returned from inside a normalized text segment,
      * where handleGetTrailSurrogate() will return the matching trail surrogate.
      */
-    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode);
+    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode) override;
 
-    virtual UBool foundNULTerminator();
+    virtual UBool foundNULTerminator() override;
 
-    virtual UBool forbidSurrogateCodePoints() const;
+    virtual UBool forbidSurrogateCodePoints() const override;
 
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
     const uint8_t *u8;
     int32_t pos;
@@ -87,24 +87,24 @@ public:
 
     virtual ~FCDUTF8CollationIterator();
 
-    virtual void resetToOffset(int32_t newOffset);
+    virtual void resetToOffset(int32_t newOffset) override;
 
-    virtual int32_t getOffset() const;
+    virtual int32_t getOffset() const override;
 
-    virtual UChar32 nextCodePoint(UErrorCode &errorCode);
+    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
 
-    virtual UChar32 previousCodePoint(UErrorCode &errorCode);
+    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;
 
 protected:
-    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode);
+    virtual uint32_t handleNextCE32(UChar32 &c, UErrorCode &errorCode) override;
 
-    virtual UChar handleGetTrailSurrogate();
+    virtual UChar handleGetTrailSurrogate() override;
 
-    virtual UBool foundNULTerminator();
+    virtual UBool foundNULTerminator() override;
 
-    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
-    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode);
+    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
 
 private:
     UBool nextHasLccc() const;
diff --git a/icu4c/source/i18n/utrans.cpp b/icu4c/source/i18n/utrans.cpp
index 29013ead125..1cbba81d658 100644
--- a/icu4c/source/i18n/utrans.cpp
+++ b/icu4c/source/i18n/utrans.cpp
@@ -52,13 +52,13 @@ public:
 
     virtual void handleReplaceBetween(int32_t start,
                                       int32_t limit,
-                                      const UnicodeString& text);
+                                      const UnicodeString& text) override;
 
     virtual void extractBetween(int32_t start,
                                 int32_t limit,
-                                UnicodeString& target) const;
+                                UnicodeString& target) const override;
 
-    virtual void copy(int32_t start, int32_t limit, int32_t dest);
+    virtual void copy(int32_t start, int32_t limit, int32_t dest) override;
 
     // virtual Replaceable *clone() const { return NULL; } same as default
 
@@ -67,7 +67,7 @@ public:
      *
      * @draft ICU 2.2
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -78,11 +78,11 @@ public:
 
 protected:
 
-    virtual int32_t getLength() const;
+    virtual int32_t getLength() const override;
 
-    virtual UChar getCharAt(int32_t offset) const;
+    virtual UChar getCharAt(int32_t offset) const override;
 
-    virtual UChar32 getChar32At(int32_t offset) const;
+    virtual UChar32 getChar32At(int32_t offset) const override;
 };
 
 UOBJECT_DEFINE_RTTI_IMPLEMENTATION(ReplaceableGlue)
diff --git a/icu4c/source/tools/ctestfw/unicode/datamap.h b/icu4c/source/tools/ctestfw/unicode/datamap.h
index 56e90763e61..b4f7f82fd64 100644
--- a/icu4c/source/tools/ctestfw/unicode/datamap.h
+++ b/icu4c/source/tools/ctestfw/unicode/datamap.h
@@ -121,16 +121,16 @@ public:
 
   virtual const ResourceBundle *getItem(const char* key, UErrorCode &status) const;
 
-  virtual const UnicodeString getString(const char* key, UErrorCode &status) const;
-  virtual int32_t getInt28(const char* key, UErrorCode &status) const;
-  virtual uint32_t getUInt28(const char* key, UErrorCode &status) const;
-  virtual const int32_t *getIntVector(int32_t &length, const char *key, UErrorCode &status) const;
-  virtual const uint8_t *getBinary(int32_t &length, const char *key, UErrorCode &status) const;
+  virtual const UnicodeString getString(const char* key, UErrorCode &status) const override;
+  virtual int32_t getInt28(const char* key, UErrorCode &status) const override;
+  virtual uint32_t getUInt28(const char* key, UErrorCode &status) const override;
+  virtual const int32_t *getIntVector(int32_t &length, const char *key, UErrorCode &status) const override;
+  virtual const uint8_t *getBinary(int32_t &length, const char *key, UErrorCode &status) const override;
 
-  virtual int32_t getInt(const char* key, UErrorCode &status) const;
+  virtual int32_t getInt(const char* key, UErrorCode &status) const override;
   
-  virtual const UnicodeString* getStringArray(int32_t& count, const char* key, UErrorCode &status) const;
-  virtual const int32_t* getIntArray(int32_t& count, const char* key, UErrorCode &status) const;
+  virtual const UnicodeString* getStringArray(int32_t& count, const char* key, UErrorCode &status) const override;
+  virtual const int32_t* getIntArray(int32_t& count, const char* key, UErrorCode &status) const override;
 
   // ... etc ...
 };
diff --git a/icu4c/source/tools/ctestfw/unicode/testdata.h b/icu4c/source/tools/ctestfw/unicode/testdata.h
index 69fa6644f33..614e36a14d6 100644
--- a/icu4c/source/tools/ctestfw/unicode/testdata.h
+++ b/icu4c/source/tools/ctestfw/unicode/testdata.h
@@ -103,10 +103,10 @@ private:
 public:
   virtual ~RBTestData();
 
-  virtual UBool getInfo(const DataMap *& info, UErrorCode &status) const;
+  virtual UBool getInfo(const DataMap *& info, UErrorCode &status) const override;
 
-  virtual UBool nextSettings(const DataMap *& settings, UErrorCode &status);
-  virtual UBool nextCase(const DataMap *& nextCase, UErrorCode &status);
+  virtual UBool nextSettings(const DataMap *& settings, UErrorCode &status) override;
+  virtual UBool nextCase(const DataMap *& nextCase, UErrorCode &status) override;
 };
 
 #endif
diff --git a/icu4c/source/tools/ctestfw/unicode/testlog.h b/icu4c/source/tools/ctestfw/unicode/testlog.h
index 9db35b9ca85..1392335270b 100644
--- a/icu4c/source/tools/ctestfw/unicode/testlog.h
+++ b/icu4c/source/tools/ctestfw/unicode/testlog.h
@@ -49,7 +49,7 @@ public:
     void setScope(const UnicodeString& message);
 
 protected:
-    virtual void handleFailure() const;
+    virtual void handleFailure() const override;
 
 private:
     TestLog &testClass;
diff --git a/icu4c/source/tools/ctestfw/unicode/tstdtmod.h b/icu4c/source/tools/ctestfw/unicode/tstdtmod.h
index 6cdce88ed6d..683a26c3398 100644
--- a/icu4c/source/tools/ctestfw/unicode/tstdtmod.h
+++ b/icu4c/source/tools/ctestfw/unicode/tstdtmod.h
@@ -92,10 +92,10 @@ public:
   RBTestDataModule(const char* name, TestLog& log, UErrorCode& status);
 
 public:
-  virtual UBool getInfo(const DataMap *& info, UErrorCode &status) const;
+  virtual UBool getInfo(const DataMap *& info, UErrorCode &status) const override;
 
-  virtual TestData* createTestData(int32_t index, UErrorCode &status) const;
-  virtual TestData* createTestData(const char* name, UErrorCode &status) const;
+  virtual TestData* createTestData(int32_t index, UErrorCode &status) const override;
+  virtual TestData* createTestData(const char* name, UErrorCode &status) const override;
 
 private:
   UResourceBundle *getTestBundle(const char* bundleName, UErrorCode &status);
diff --git a/icu4c/source/tools/genrb/parse.cpp b/icu4c/source/tools/genrb/parse.cpp
index 5abe5d65673..7d5ffe1fc78 100644
--- a/icu4c/source/tools/genrb/parse.cpp
+++ b/icu4c/source/tools/genrb/parse.cpp
@@ -683,7 +683,7 @@ public:
     virtual void getRules(
             const char *localeID, const char *collationType,
             UnicodeString &rules,
-            const char *&errorReason, UErrorCode &errorCode);
+            const char *&errorReason, UErrorCode &errorCode) override;
 
 private:
     const char *inputDir;
diff --git a/icu4c/source/tools/genrb/reslist.h b/icu4c/source/tools/genrb/reslist.h
index 78b916ef02d..c9731880ea6 100644
--- a/icu4c/source/tools/genrb/reslist.h
+++ b/icu4c/source/tools/genrb/reslist.h
@@ -294,9 +294,9 @@ public:
 
     void add(SResource *res);
 
-    virtual void handleWrite16(SRBRoot *bundle);
-    virtual void handlePreWrite(uint32_t *byteOffset);
-    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset);
+    virtual void handleWrite16(SRBRoot *bundle) override;
+    virtual void handlePreWrite(uint32_t *byteOffset) override;
+    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset) override;
 
     SResource *fLast;
 };
@@ -313,7 +313,7 @@ public:
 
     void add(SResource *res);
 
-    virtual void handleWrite16(SRBRoot *bundle);
+    virtual void handleWrite16(SRBRoot *bundle) override;
 };
 
 class StringBaseResource : public SResource {
@@ -329,8 +329,8 @@ public:
     const UChar *getBuffer() const { return icu::toUCharPtr(fString.getBuffer()); }
     int32_t length() const { return fString.length(); }
 
-    virtual void handlePreWrite(uint32_t *byteOffset);
-    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset);
+    virtual void handlePreWrite(uint32_t *byteOffset) override;
+    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset) override;
 
     // TODO: private with getter?
     icu::UnicodeString fString;
@@ -363,8 +363,8 @@ public:
         return fNumCharsForLength + length() + 1;  // +1 for the NUL
     }
 
-    virtual void handlePreflightStrings(SRBRoot *bundle, UHashtable *stringSet, UErrorCode &errorCode);
-    virtual void handleWrite16(SRBRoot *bundle);
+    virtual void handlePreflightStrings(SRBRoot *bundle, UHashtable *stringSet, UErrorCode &errorCode) override;
+    virtual void handleWrite16(SRBRoot *bundle) override;
 
     void writeUTF16v2(int32_t base, icu::UnicodeString &dest);
 
@@ -401,8 +401,8 @@ public:
 
     void add(int32_t value, UErrorCode &errorCode);
 
-    virtual void handlePreWrite(uint32_t *byteOffset);
-    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset);
+    virtual void handlePreWrite(uint32_t *byteOffset) override;
+    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset) override;
 
     // TODO: UVector32
     size_t fCount;
@@ -417,8 +417,8 @@ public:
                    const UString* comment, UErrorCode &errorCode);
     virtual ~BinaryResource();
 
-    virtual void handlePreWrite(uint32_t *byteOffset);
-    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset);
+    virtual void handlePreWrite(uint32_t *byteOffset) override;
+    virtual void handleWrite(UNewDataMemory *mem, uint32_t *byteOffset) override;
 
     // TODO: CharString?
     uint32_t fLength;
diff --git a/icu4c/source/tools/toolutil/toolutil.h b/icu4c/source/tools/toolutil/toolutil.h
index 00a9c364b69..1c9f06758ff 100644
--- a/icu4c/source/tools/toolutil/toolutil.h
+++ b/icu4c/source/tools/toolutil/toolutil.h
@@ -48,7 +48,7 @@ public:
     IcuToolErrorCode(const char *loc) : location(loc) {}
     virtual ~IcuToolErrorCode();
 protected:
-    virtual void handleFailure() const;
+    virtual void handleFailure() const override;
 private:
     const char *location;
 };
diff --git a/icu4c/source/tools/toolutil/xmlparser.h b/icu4c/source/tools/toolutil/xmlparser.h
index 75c8ed7e53d..5a3a24c5ed9 100644
--- a/icu4c/source/tools/toolutil/xmlparser.h
+++ b/icu4c/source/tools/toolutil/xmlparser.h
@@ -116,7 +116,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
@@ -188,7 +188,7 @@ public:
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
      */
-    virtual UClassID getDynamicClassID() const;
+    virtual UClassID getDynamicClassID() const override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.