diff --git a/icu4c/source/i18n/number_fluent.cpp b/icu4c/source/i18n/number_fluent.cpp
index 687adb6b5ba..06b10fd2765 100644
--- a/icu4c/source/i18n/number_fluent.cpp
+++ b/icu4c/source/i18n/number_fluent.cpp
@@ -776,7 +776,7 @@ Appendable& FormattedNumber::appendTo(Appendable& appendable) {
     return appendTo(appendable, localStatus);
 }
 
-Appendable& FormattedNumber::appendTo(Appendable& appendable, UErrorCode& status) {
+Appendable& FormattedNumber::appendTo(Appendable& appendable, UErrorCode& status) const {
     if (U_FAILURE(status)) {
         return appendable;
     }
diff --git a/icu4c/source/i18n/unicode/numberformatter.h b/icu4c/source/i18n/unicode/numberformatter.h
index 3ab08319f73..4dd03ce8472 100644
--- a/icu4c/source/i18n/unicode/numberformatter.h
+++ b/icu4c/source/i18n/unicode/numberformatter.h
@@ -2430,7 +2430,7 @@ class U_I18N_API FormattedNumber : public UMemory {
      * @draft ICU 62
      * @see Appendable
      */
-    Appendable &appendTo(Appendable &appendable, UErrorCode& status);
+    Appendable &appendTo(Appendable &appendable, UErrorCode& status) const;
 
 #ifndef U_HIDE_DEPRECATED_API
     /**