diff --git a/icu4c/source/common/unicode/brkiter.h b/icu4c/source/common/unicode/brkiter.h
index 79b890684c7..0e6f229320c 100644
--- a/icu4c/source/common/unicode/brkiter.h
+++ b/icu4c/source/common/unicode/brkiter.h
@@ -146,7 +146,7 @@ public:
      * will return distinct unequal values.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
 
     /**
      * Return a CharacterIterator over the text being analyzed.
diff --git a/icu4c/source/common/unicode/chariter.h b/icu4c/source/common/unicode/chariter.h
index 3c66c4d93ea..4f320b90e2c 100644
--- a/icu4c/source/common/unicode/chariter.h
+++ b/icu4c/source/common/unicode/chariter.h
@@ -142,7 +142,7 @@ public:
      * @return a UClassID for this ForwardCharacterIterator 
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
     
     /**
      * Gets the current code unit for returning and advances to the next code unit
diff --git a/icu4c/source/common/unicode/unifilt.h b/icu4c/source/common/unicode/unifilt.h
index 7870b559391..0fcaf4b789c 100644
--- a/icu4c/source/common/unicode/unifilt.h
+++ b/icu4c/source/common/unicode/unifilt.h
@@ -77,7 +77,7 @@ public:
      * @return clone, or nullptr if an error occurred
      * @stable ICU 2.4
      */
-    virtual UnicodeFilter* clone() const = 0;
+    virtual UnicodeFilter* clone() const override = 0;
 
     /**
      * Returns <tt>true</tt> for characters that are in the selected
@@ -93,7 +93,7 @@ public:
      * and return the pointer.
      * @stable ICU 2.4
      */
-    virtual UnicodeMatcher* toMatcher() const;
+    virtual UnicodeMatcher* toMatcher() const override;
 
     /**
      * Implement UnicodeMatcher API.
@@ -102,13 +102,13 @@ public:
     virtual UMatchDegree matches(const Replaceable& text,
                                  int32_t& offset,
                                  int32_t limit,
-                                 UBool incremental);
+                                 UBool incremental) override;
 
     /**
      * UnicodeFunctor API.  Nothing to do.
      * @stable ICU 2.4
      */
-    virtual void setData(const TransliterationRuleData*);
+    virtual void setData(const TransliterationRuleData*) override;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for this class.
diff --git a/icu4c/source/common/unicode/unifunct.h b/icu4c/source/common/unicode/unifunct.h
index 482794175f6..8751302494b 100644
--- a/icu4c/source/common/unicode/unifunct.h
+++ b/icu4c/source/common/unicode/unifunct.h
@@ -100,7 +100,7 @@ public:
      * different class IDs.
      * @stable ICU 2.4
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
 
     /**
      * Set the data object associated with this functor.  The data
diff --git a/icu4c/source/i18n/casetrn.h b/icu4c/source/i18n/casetrn.h
index 2ec3e736deb..a00480db60b 100644
--- a/icu4c/source/i18n/casetrn.h
+++ b/icu4c/source/i18n/casetrn.h
@@ -58,7 +58,7 @@ public:
      * Transliterator API.
      * @return a copy of the object.
      */
-    virtual CaseMapTransliterator* clone() const = 0;
+    virtual CaseMapTransliterator* clone() const override = 0;
 
     /**
      * ICU "poor man's RTTI", returns a UClassID for the actual class.
@@ -82,7 +82,7 @@ protected:
      */
     virtual void handleTransliterate(Replaceable& text,
                                      UTransPosition& offsets, 
-                                     UBool isIncremental) const;
+                                     UBool isIncremental) const override;
 
     UCaseMapFull *fMap;
 
diff --git a/icu4c/source/i18n/csrmbcs.h b/icu4c/source/i18n/csrmbcs.h
index 8ccf1d56a95..ff7fc4e2a78 100644
--- a/icu4c/source/i18n/csrmbcs.h
+++ b/icu4c/source/i18n/csrmbcs.h
@@ -71,9 +71,9 @@ public:
      * @return the charset name.
      */
 
-    const char *getName() const = 0;
-    const char *getLanguage() const = 0;
-    UBool match(InputText* input, CharsetMatch *results) const = 0;
+    const char *getName() const override = 0;
+    const char *getLanguage() const override = 0;
+    UBool match(InputText* input, CharsetMatch *results) const override = 0;
 
     /**
      * Get the next character (however many bytes it is) from the input data
@@ -100,12 +100,12 @@ class CharsetRecog_sjis : public CharsetRecog_mbcs {
 public:
     virtual ~CharsetRecog_sjis();
 
-    UBool nextChar(IteratedChar *it, InputText *det) const;
+    UBool nextChar(IteratedChar *it, InputText *det) const override;
 
-    UBool match(InputText* input, CharsetMatch *results) const;
+    UBool match(InputText* input, CharsetMatch *results) const override;
 
-    const char *getName() const;
-    const char *getLanguage() const;
+    const char *getName() const override;
+    const char *getLanguage() const override;
 
 };
 
@@ -121,17 +121,17 @@ class CharsetRecog_euc : public CharsetRecog_mbcs
 public:
     virtual ~CharsetRecog_euc();
 
-    const char *getName() const = 0;
-    const char *getLanguage() const = 0;
+    const char *getName() const override = 0;
+    const char *getLanguage() const override = 0;
 
-    UBool match(InputText* input, CharsetMatch *results) const = 0;
+    UBool match(InputText* input, CharsetMatch *results) const override = 0;
     /*
      *  (non-Javadoc)
      *  Get the next character value for EUC based encodings.
      *  Character "value" is simply the raw bytes that make up the character
      *     packed into an int.
      */
-    UBool nextChar(IteratedChar *it, InputText *det) const;
+    UBool nextChar(IteratedChar *it, InputText *det) const override;
 };
 
 /**
@@ -143,10 +143,10 @@ class CharsetRecog_euc_jp : public CharsetRecog_euc
 public:
     virtual ~CharsetRecog_euc_jp();
 
-    const char *getName() const;
-    const char *getLanguage() const;
+    const char *getName() const override;
+    const char *getLanguage() const override;
 
-    UBool match(InputText* input, CharsetMatch *results) const;
+    UBool match(InputText* input, CharsetMatch *results) const override;
 };
 
 /**
@@ -158,10 +158,10 @@ class CharsetRecog_euc_kr : public CharsetRecog_euc
 public:
     virtual ~CharsetRecog_euc_kr();
 
-    const char *getName() const;
-    const char *getLanguage() const;
+    const char *getName() const override;
+    const char *getLanguage() const override;
 
-    UBool match(InputText* input, CharsetMatch *results) const;
+    UBool match(InputText* input, CharsetMatch *results) const override;
 };
 
 /**
@@ -174,12 +174,12 @@ class CharsetRecog_big5 : public CharsetRecog_mbcs
 public:
     virtual ~CharsetRecog_big5();
 
-    UBool nextChar(IteratedChar* it, InputText* det) const;
+    UBool nextChar(IteratedChar* it, InputText* det) const override;
 
-    const char *getName() const;
-    const char *getLanguage() const;
+    const char *getName() const override;
+    const char *getLanguage() const override;
 
-    UBool match(InputText* input, CharsetMatch *results) const;
+    UBool match(InputText* input, CharsetMatch *results) const override;
 };
 
 
@@ -193,12 +193,12 @@ class CharsetRecog_gb_18030 : public CharsetRecog_mbcs
 public:
     virtual ~CharsetRecog_gb_18030();
 
-    UBool nextChar(IteratedChar* it, InputText* det) const;
+    UBool nextChar(IteratedChar* it, InputText* det) const override;
 
-    const char *getName() const;
-    const char *getLanguage() const;
+    const char *getName() const override;
+    const char *getLanguage() const override;
 
-    UBool match(InputText* input, CharsetMatch *results) const;
+    UBool match(InputText* input, CharsetMatch *results) const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/csrsbcs.h b/icu4c/source/i18n/csrsbcs.h
index 97687836795..18908efe71e 100644
--- a/icu4c/source/i18n/csrsbcs.h
+++ b/icu4c/source/i18n/csrsbcs.h
@@ -74,8 +74,8 @@ class CharsetRecog_sbcs : public CharsetRecognizer
 public:
     CharsetRecog_sbcs();
     virtual ~CharsetRecog_sbcs();
-    virtual const char *getName() const = 0;
-    virtual UBool match(InputText *det, CharsetMatch *results) const = 0;
+    virtual const char *getName() const override = 0;
+    virtual UBool match(InputText *det, CharsetMatch *results) const override = 0;
     virtual int32_t match_sbcs(InputText *det, const int32_t ngrams[], const uint8_t charMap[]) const;
 };
 
diff --git a/icu4c/source/i18n/csrucode.h b/icu4c/source/i18n/csrucode.h
index cef35447fb7..78e08d22f11 100644
--- a/icu4c/source/i18n/csrucode.h
+++ b/icu4c/source/i18n/csrucode.h
@@ -33,12 +33,12 @@ public:
     /* (non-Javadoc)
      * @see com.ibm.icu.text.CharsetRecognizer#getName()
      */
-    const char* getName() const = 0;
+    const char* getName() const override = 0;
 
     /* (non-Javadoc)
      * @see com.ibm.icu.text.CharsetRecognizer#match(com.ibm.icu.text.CharsetDetector)
      */
-    UBool match(InputText* textIn, CharsetMatch *results) const = 0;
+    UBool match(InputText* textIn, CharsetMatch *results) const override = 0;
 };
 
 
@@ -48,9 +48,9 @@ public:
 
     virtual ~CharsetRecog_UTF_16_BE();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    UBool match(InputText* textIn, CharsetMatch *results) const;
+    UBool match(InputText* textIn, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_UTF_16_LE : public CharsetRecog_Unicode
@@ -59,9 +59,9 @@ public:
 
     virtual ~CharsetRecog_UTF_16_LE();
 
-    const char *getName() const;
+    const char *getName() const override;
 
-    UBool match(InputText* textIn, CharsetMatch *results) const;
+    UBool match(InputText* textIn, CharsetMatch *results) const override;
 };
 
 class CharsetRecog_UTF_32 : public CharsetRecog_Unicode
@@ -72,34 +72,34 @@ public:
 
     virtual ~CharsetRecog_UTF_32();
 
-    const char* getName() const = 0;
+    const char* getName() const override = 0;
 
-    UBool match(InputText* textIn, CharsetMatch *results) const;
+    UBool match(InputText* textIn, CharsetMatch *results) const override;
 };
 
 
 class CharsetRecog_UTF_32_BE : public CharsetRecog_UTF_32
 {
 protected:
-    int32_t getChar(const uint8_t *input, int32_t index) const;
+    int32_t getChar(const uint8_t *input, int32_t index) const override;
 
 public:
 
     virtual ~CharsetRecog_UTF_32_BE();
 
-    const char *getName() const;
+    const char *getName() const override;
 };
 
 
 class CharsetRecog_UTF_32_LE : public CharsetRecog_UTF_32
 {
 protected:
-    int32_t getChar(const uint8_t *input, int32_t index) const;
+    int32_t getChar(const uint8_t *input, int32_t index) const override;
 
 public:
     virtual ~CharsetRecog_UTF_32_LE();
 
-    const char* getName() const;
+    const char* getName() const override;
 };
 
 U_NAMESPACE_END
diff --git a/icu4c/source/i18n/unicode/basictz.h b/icu4c/source/i18n/unicode/basictz.h
index 64be77c5807..6c94ad3bedb 100644
--- a/icu4c/source/i18n/unicode/basictz.h
+++ b/icu4c/source/i18n/unicode/basictz.h
@@ -49,7 +49,7 @@ public:
      * @return clone, or nullptr if an error occurred
      * @stable ICU 3.8
      */
-    virtual BasicTimeZone* clone() const = 0;
+    virtual BasicTimeZone* clone() const override = 0;
 
     /**
      * Gets the first time zone transition after the base time.
diff --git a/icu4c/source/i18n/unicode/calendar.h b/icu4c/source/i18n/unicode/calendar.h
index 6e3526eb27e..c1bdf928203 100644
--- a/icu4c/source/i18n/unicode/calendar.h
+++ b/icu4c/source/i18n/unicode/calendar.h
@@ -1341,7 +1341,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
 
     /**
      * Returns the calendar type name string for this Calendar object.
diff --git a/icu4c/source/i18n/unicode/coll.h b/icu4c/source/i18n/unicode/coll.h
index c9a2484768a..9e0050ba6ce 100644
--- a/icu4c/source/i18n/unicode/coll.h
+++ b/icu4c/source/i18n/unicode/coll.h
@@ -864,7 +864,7 @@ public:
      *         IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
 
     /**
      * Universal attribute setter
diff --git a/icu4c/source/i18n/unicode/datefmt.h b/icu4c/source/i18n/unicode/datefmt.h
index 892c823609b..027fb1712f4 100644
--- a/icu4c/source/i18n/unicode/datefmt.h
+++ b/icu4c/source/i18n/unicode/datefmt.h
@@ -229,13 +229,13 @@ public:
      * @return clone, or nullptr if an error occurred
      * @stable ICU 2.0
      */
-    virtual DateFormat* clone() const = 0;
+    virtual DateFormat* clone() const override = 0;
 
     /**
      * Equality operator.  Returns true if the two formats have the same behavior.
      * @stable ICU 2.0
      */
-    virtual bool operator==(const Format&) const;
+    virtual bool operator==(const Format&) const override;
 
 
     using Format::format;
@@ -257,7 +257,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPosition& pos,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
 
     /**
      * Format an object to produce a string. This method handles Formattable
@@ -277,7 +277,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPositionIterator* posIter,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
     /**
      * Formats a date into a date/time string. This is an abstract method which
      * concrete subclasses must implement.
@@ -523,7 +523,7 @@ public:
      */
     virtual void parseObject(const UnicodeString& source,
                              Formattable& result,
-                             ParsePosition& parse_pos) const;
+                             ParsePosition& parse_pos) const override;
 
     /**
      * Create a default date/time formatter that uses the SHORT style for both
diff --git a/icu4c/source/i18n/unicode/numfmt.h b/icu4c/source/i18n/unicode/numfmt.h
index 8cf226bfa3a..135fe5b6609 100644
--- a/icu4c/source/i18n/unicode/numfmt.h
+++ b/icu4c/source/i18n/unicode/numfmt.h
@@ -266,7 +266,7 @@ public:
      * @return clone, or nullptr if an error occurred
      * @stable ICU 2.0
      */
-    virtual NumberFormat* clone() const = 0;
+    virtual NumberFormat* clone() const override = 0;
 
     /**
      * Return true if the given Format objects are semantically equal.
@@ -274,7 +274,7 @@ 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 Format::format;
@@ -297,7 +297,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPosition& pos,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
 
     /**
      * Format an object to produce a string.  This method handles
@@ -318,7 +318,7 @@ public:
     virtual UnicodeString& format(const Formattable& obj,
                                   UnicodeString& appendTo,
                                   FieldPositionIterator* posIter,
-                                  UErrorCode& status) const;
+                                  UErrorCode& status) const override;
 
     /**
      * Parse a string to produce an object.  This methods handles
@@ -350,7 +350,7 @@ public:
      */
     virtual void parseObject(const UnicodeString& source,
                              Formattable& result,
-                             ParsePosition& parse_pos) const;
+                             ParsePosition& parse_pos) const override;
 
     /**
      * Format a double number. These methods call the NumberFormat
@@ -1077,7 +1077,7 @@ public:
      * other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
 
 protected:
 
@@ -1238,12 +1238,12 @@ public:
     /**
      * @stable ICU 2.6
      */
-    virtual UBool visible(void) const;
+    virtual UBool visible(void) const override;
 
     /**
      * @stable ICU 2.6
      */
-    virtual const UnicodeString * getSupportedIDs(int32_t &count, UErrorCode& status) const;
+    virtual const UnicodeString * getSupportedIDs(int32_t &count, UErrorCode& status) const override;
 };
 #endif /* #if !UCONFIG_NO_SERVICE */
 
diff --git a/icu4c/source/i18n/unicode/timezone.h b/icu4c/source/i18n/unicode/timezone.h
index 762caac94c7..382ad97d5bf 100644
--- a/icu4c/source/i18n/unicode/timezone.h
+++ b/icu4c/source/i18n/unicode/timezone.h
@@ -839,7 +839,7 @@ public:
      *           same class ID. Objects of other classes have different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
     
     /**
      * Returns the amount of time to be added to local standard time
diff --git a/icu4c/source/i18n/unicode/translit.h b/icu4c/source/i18n/unicode/translit.h
index 525f2b0056e..56eb6448ca4 100644
--- a/icu4c/source/i18n/unicode/translit.h
+++ b/icu4c/source/i18n/unicode/translit.h
@@ -1527,7 +1527,7 @@ public:
      * different class IDs.
      * @stable ICU 2.0
      */
-    virtual UClassID getDynamicClassID(void) const = 0;
+    virtual UClassID getDynamicClassID(void) const override = 0;
 
 private:
     static UBool initializeRegistry(UErrorCode &status);