diff --git a/Lib/test/test_cext/__init__.py b/Lib/test/test_cext/__init__.py
index 730b91abfb2715..77929495336638 100644
--- a/Lib/test/test_cext/__init__.py
+++ b/Lib/test/test_cext/__init__.py
@@ -1,5 +1,7 @@
 # gh-116869: Build a basic C test extension to check that the Python C API
 # does not emit C compiler warnings.
+#
+# Python C API must build with -Werror=declaration-after-statement.
 
 import os.path
 import shutil
diff --git a/Lib/test/test_cext/setup.py b/Lib/test/test_cext/setup.py
index dd57a5fb8e1e1b..aed7e6d46dd8d0 100644
--- a/Lib/test/test_cext/setup.py
+++ b/Lib/test/test_cext/setup.py
@@ -16,6 +16,10 @@
         # The purpose of test_cext extension is to check that building a C
         # extension using the Python C API does not emit C compiler warnings.
         '-Werror',
+
+        # gh-116869: The Python C API must build with
+        # -Werror=declaration-after-statement.
+        '-Werror=declaration-after-statement',
     ]
 else:
     # Don't pass any compiler flag to MSVC