diff --git a/libcxx/test/CMakeLists.txt b/libcxx/test/CMakeLists.txt
--- a/libcxx/test/CMakeLists.txt
+++ b/libcxx/test/CMakeLists.txt
@@ -83,6 +83,8 @@
     "TargetInfo to use when setting up test environment.")
 set(LIBCXX_EXECUTOR "${Python3_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/../utils/run.py" CACHE STRING
     "Executor to use when running tests.")
+set(LIBCXX_ADDITIONAL_CONFIG "" CACHE STRING
+    "Additional config to load when setting up test environment.")
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")
 
diff --git a/libcxx/test/lit.site.cfg.in b/libcxx/test/lit.site.cfg.in
--- a/libcxx/test/lit.site.cfg.in
+++ b/libcxx/test/lit.site.cfg.in
@@ -39,6 +39,10 @@
 # Code signing
 config.llvm_codesign_identity   = "@LLVM_CODESIGNING_IDENTITY@"
 
+# Allow the user to inject additional config parameters at this point
+if bool("@LIBCXX_ADDITIONAL_CONFIG@"):
+    lit_config.load_config(config, "@LIBCXX_ADDITIONAL_CONFIG@")
+
 # Let the main config do the real work.
 config.loaded_site_config = True
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")