diff --git a/lib/Platform/Intl/PlatformIntlApple.mm b/lib/Platform/Intl/PlatformIntlApple.mm
index 324f6a34972..099499220f5 100644
--- a/lib/Platform/Intl/PlatformIntlApple.mm
+++ b/lib/Platform/Intl/PlatformIntlApple.mm
@@ -42,6 +42,10 @@
       std::replace(u16str.begin(), u16str.end(), u'_', u'-');
       // Some locales may still not be properly canonicalized (e.g. en_US_POSIX
       // should be en-US-posix).
+      // Note that we do not need to handle legacy extensions here (unlike
+      // getDefaultLocale), since the documentation for
+      // availableLocaleIdentifiers states that they will only contain a
+      // language, country, and script code.
       if (auto parsed = ParsedLocaleIdentifier::parse(u16str))
         vec->push_back(parsed->canonicalize());
     }
@@ -53,12 +57,17 @@
   static const std::u16string *defLocale = new std::u16string([] {
     // Environment variable used for testing only
     const char *testLocale = std::getenv("_HERMES_TEST_LOCALE");
-    if (testLocale) {
-      NSString *nsTestLocale = [NSString stringWithUTF8String:testLocale];
-      return nsStringToU16String(nsTestLocale);
-    }
-    NSString *nsDefLocale = [[NSLocale currentLocale] localeIdentifier];
-    auto defLocale = nsStringToU16String(nsDefLocale);
+    NSString *nsLocale = testLocale
+        ? [NSString stringWithUTF8String:testLocale]
+        : [[NSLocale currentLocale] localeIdentifier];
+    auto defLocale = nsStringToU16String(nsLocale);
+
+    // The locale identifier may occasionally contain legacy style locale
+    // extensions. We cannot handle them so remove them before parsing the tag.
+    size_t delimIdx = defLocale.find(u'@');
+    if (delimIdx != std::u16string::npos)
+      defLocale.resize(delimIdx);
+
     // See the comment in getAvailableLocales.
     std::replace(defLocale.begin(), defLocale.end(), u'_', u'-');
     if (auto parsed = ParsedLocaleIdentifier::parse(defLocale))
diff --git a/test/hermes/intl/default-locale-legacy-extension.js b/test/hermes/intl/default-locale-legacy-extension.js
new file mode 100644
index 00000000000..01eaee23635
--- /dev/null
+++ b/test/hermes/intl/default-locale-legacy-extension.js
@@ -0,0 +1,15 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// RUN: _HERMES_TEST_LOCALE="en-GB@foo=bar" %hermes %s | %FileCheck --match-full-lines %s
+// REQUIRES: intl
+
+print(Intl.Collator().resolvedOptions().locale);
+// CHECK: en-GB
+
+print(Intl.DateTimeFormat().resolvedOptions().locale);
+// CHECK-NEXT: en-GB