diff --git a/libcxx/utils/libcxx/test/params.py b/libcxx/utils/libcxx/test/params.py
--- a/libcxx/utils/libcxx/test/params.py
+++ b/libcxx/utils/libcxx/test/params.py
@@ -76,6 +76,7 @@
             actions=lambda modules: [
               AddFeature('modules-build'),
               AddCompileFlag('-fmodules'),
+              AddCompileFlag('-fcxx-modules'), # AppleClang disregards -fmodules entirely when compiling C++. This enables modules for C++.
             ] if modules else []),
 
   Parameter(name='enable_exceptions', choices=[True, False], type=bool, default=True,