diff --git a/icu4c/source/common/unicode/localpointer.h b/icu4c/source/common/unicode/localpointer.h
index 487ddb48b78..9c891bf3088 100644
--- a/icu4c/source/common/unicode/localpointer.h
+++ b/icu4c/source/common/unicode/localpointer.h
@@ -21,7 +21,7 @@
 
 /**
  * \file
- * \brief C++ header-only API: "Smart pointers" for use with and in ICU4C C++ code.
+ * \brief C++ API: "Smart pointers" for use with and in ICU4C C++ code.
  *
  * These classes are inspired by
  * - std::auto_ptr
@@ -40,11 +40,11 @@
 
 #include "unicode/utypes.h"
 
-#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+#if U_SHOW_CPLUSPLUS_API
 
 #include <memory>
 
-namespace U_ICU_NAMESPACE_OR_INTERNAL {
+U_NAMESPACE_BEGIN
 
 /**
  * "Smart pointer" base class; do not use directly: use LocalPointer etc.
@@ -603,7 +603,7 @@ public:
 }  // namespace internal
 #endif
 
-}  // U_ICU_NAMESPACE_OR_INTERNAL
+U_NAMESPACE_END
 
-#endif  // U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+#endif  /* U_SHOW_CPLUSPLUS_API */
 #endif  /* __LOCALPOINTER_H__ */
diff --git a/icu4c/source/common/unicode/normlzr.h b/icu4c/source/common/unicode/normlzr.h
index 40848bf00ca..0309bce5382 100644
--- a/icu4c/source/common/unicode/normlzr.h
+++ b/icu4c/source/common/unicode/normlzr.h
@@ -801,8 +801,8 @@ Normalizer::compare(const UnicodeString &s1, const UnicodeString &s2,
                     uint32_t options,
                     UErrorCode &errorCode) {
   // all argument checking is done in unorm_compare
-  return unorm_compare(U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(s1.getBuffer()), s1.length(),
-                       U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(s2.getBuffer()), s2.length(),
+  return unorm_compare(toUCharPtr(s1.getBuffer()), s1.length(),
+                       toUCharPtr(s2.getBuffer()), s2.length(),
                        options,
                        &errorCode);
 }
diff --git a/icu4c/source/common/unicode/unistr.h b/icu4c/source/common/unicode/unistr.h
index a1901c91504..39d789fd6ef 100644
--- a/icu4c/source/common/unicode/unistr.h
+++ b/icu4c/source/common/unicode/unistr.h
@@ -4676,7 +4676,7 @@ UnicodeString::startsWith(const UnicodeString& srcText,
 inline UBool
 UnicodeString::startsWith(ConstChar16Ptr srcChars, int32_t srcLength) const {
   if(srcLength < 0) {
-    srcLength = u_strlen(U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(srcChars));
+    srcLength = u_strlen(toUCharPtr(srcChars));
   }
   return doEqualsSubstring(0, srcLength, srcChars, 0, srcLength);
 }
@@ -4684,7 +4684,7 @@ UnicodeString::startsWith(ConstChar16Ptr srcChars, int32_t srcLength) const {
 inline UBool
 UnicodeString::startsWith(const char16_t *srcChars, int32_t srcStart, int32_t srcLength) const {
   if(srcLength < 0) {
-    srcLength = u_strlen(U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(srcChars));
+    srcLength = u_strlen(toUCharPtr(srcChars));
   }
   return doEqualsSubstring(0, srcLength, srcChars, srcStart, srcLength);
 }
@@ -4707,7 +4707,7 @@ inline UBool
 UnicodeString::endsWith(ConstChar16Ptr srcChars,
             int32_t srcLength) const {
   if(srcLength < 0) {
-    srcLength = u_strlen(U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(srcChars));
+    srcLength = u_strlen(toUCharPtr(srcChars));
   }
   return doEqualsSubstring(length() - srcLength, srcLength, srcChars, 0, srcLength);
 }
@@ -4717,7 +4717,7 @@ UnicodeString::endsWith(const char16_t *srcChars,
             int32_t srcStart,
             int32_t srcLength) const {
   if(srcLength < 0) {
-    srcLength = u_strlen(U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(srcChars + srcStart));
+    srcLength = u_strlen(toUCharPtr(srcChars + srcStart));
   }
   return doEqualsSubstring(length() - srcLength, srcLength,
                    srcChars, srcStart, srcLength);
diff --git a/icu4c/source/common/unicode/uset.h b/icu4c/source/common/unicode/uset.h
index 345d6244544..b5333a5f5ce 100644
--- a/icu4c/source/common/unicode/uset.h
+++ b/icu4c/source/common/unicode/uset.h
@@ -346,9 +346,9 @@ uset_openPatternOptions(const UChar* pattern, int32_t patternLength,
 U_CAPI void U_EXPORT2
 uset_close(USet* set);
 
-#if U_SHOW_CPLUSPLUS_API || U_SHOW_CPLUSPLUS_HEADER_API
+#if U_SHOW_CPLUSPLUS_API
 
-namespace U_ICU_NAMESPACE_OR_INTERNAL {
+U_NAMESPACE_BEGIN
 
 /**
  * \class LocalUSetPointer
@@ -361,7 +361,7 @@ namespace U_ICU_NAMESPACE_OR_INTERNAL {
  */
 U_DEFINE_LOCAL_OPEN_POINTER(LocalUSetPointer, USet, uset_close);
 
-}  // U_ICU_NAMESPACE_OR_INTERNAL
+U_NAMESPACE_END
 
 #endif
 
diff --git a/icu4c/source/i18n/unicode/ucol.h b/icu4c/source/i18n/unicode/ucol.h
index 8b6dfeaa0a8..ae4f29c3c6c 100644
--- a/icu4c/source/i18n/unicode/ucol.h
+++ b/icu4c/source/i18n/unicode/ucol.h
@@ -1572,8 +1572,8 @@ class Predicate {
         return compare(
             ucol_strcoll(
                 collator,
-                U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(lhs.getBuffer()), lhs.length(),
-                U_ICU_NAMESPACE_OR_INTERNAL::toUCharPtr(rhs.getBuffer()), rhs.length()),
+                toUCharPtr(lhs.getBuffer()), lhs.length(),
+                toUCharPtr(rhs.getBuffer()), rhs.length()),
             result);
     }