diff --git a/Lib/test/test_cext/setup.py b/Lib/test/test_cext/setup.py
index ccad3fa62ad086..383d256c1eb53b 100644
--- a/Lib/test/test_cext/setup.py
+++ b/Lib/test/test_cext/setup.py
@@ -17,6 +17,9 @@
         # 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-120593: Check the 'const' qualifier
+        '-Wcast-qual',
     ]
     if not support.Py_GIL_DISABLED:
         CFLAGS.append(