diff --git a/icu4c/source/common/uloc_tag.cpp b/icu4c/source/common/uloc_tag.cpp
index 7a2b86735860..d261e46b3e28 100644
--- a/icu4c/source/common/uloc_tag.cpp
+++ b/icu4c/source/common/uloc_tag.cpp
@@ -1326,14 +1326,23 @@ _appendKeywordsToLanguageTag(const char* localeID, icu::ByteSink& sink, UBool st
                         attrBufLength = 0;
                         for (; i < len; i++) {
                             if (buf[i] != '-') {
-                                attrBuf[attrBufLength++] = buf[i];
+                                if (static_cast<size_t>(attrBufLength) < sizeof(attrBuf)) {
+                                    attrBuf[attrBufLength++] = buf[i];
+                                } else {
+                                    *status = U_ILLEGAL_ARGUMENT_ERROR;
+                                    return;
+                                }
                             } else {
                                 i++;
                                 break;
                             }
                         }
                         if (attrBufLength > 0) {
-                            attrBuf[attrBufLength] = 0;
+                            if (static_cast<size_t>(attrBufLength) < sizeof(attrBuf)) {
+                                attrBuf[attrBufLength] = 0;
+                            } else {
+                                *status = U_STRING_NOT_TERMINATED_WARNING;
+                            }
 
                         } else if (i >= len){
                             break;
diff --git a/icu4c/source/test/intltest/loctest.cpp b/icu4c/source/test/intltest/loctest.cpp
index 1c88f77fe638..d72d7acfd32a 100644
--- a/icu4c/source/test/intltest/loctest.cpp
+++ b/icu4c/source/test/intltest/loctest.cpp
@@ -5982,6 +5982,8 @@ void LocaleTest::TestToLanguageTag() {
       {"und-1994-biske-rozaj-x-private", "und-1994-biske-rozaj-x-private"},
       // ICU-22497
       {"-ins0-ins17Rz-yqyq-UWLF-uRyq-UWLF-uRRyq-UWLF-uR-UWLF-uRns0-ins17Rz-yq-UWLF-uRyq-UWLF-uRRyq-LF-uRyq-UWLF-uRRyq-UWLF-uRq-UWLF-uRyq-UWLF-uRRyq-UWLF-uR", ""},
+      // ICU-22504
+      {"@attribute=zzo9zzzzzzzs0zzzzzzzzzz55555555555555555555500000000000000000000fffffffffffffffffffffffffzzzzz2mfPAK", ""},
     };
     int32_t i;
     for (i=0; i < UPRV_LENGTHOF(testCases); i++) {