diff --git a/Include/cpython/pyatomic.h b/Include/cpython/pyatomic.h
index e85b360c986668c..18842582bf06bd0 100644
--- a/Include/cpython/pyatomic.h
+++ b/Include/cpython/pyatomic.h
@@ -582,7 +582,7 @@ static inline void _Py_atomic_fence_release(void);
 #  define Py_ATOMIC_GCC_H
 #  include "pyatomic_gcc.h"
 #  undef Py_ATOMIC_GCC_H
-#elif __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
+#elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
 #  define Py_ATOMIC_STD_H
 #  include "pyatomic_std.h"
 #  undef Py_ATOMIC_STD_H
diff --git a/Include/pyport.h b/Include/pyport.h
index 73a3e6cdaf09200..3936b999a88d505 100644
--- a/Include/pyport.h
+++ b/Include/pyport.h
@@ -491,7 +491,7 @@ extern "C" {
 #ifdef WITH_THREAD
 #  ifdef thread_local
 #    define _Py_thread_local thread_local
-#  elif __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
+#  elif defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_THREADS__)
 #    define _Py_thread_local _Thread_local
 #  elif defined(_MSC_VER)  /* AKA NT_THREADS */
 #    define _Py_thread_local __declspec(thread)
diff --git a/Lib/test/test_cppext/__init__.py b/Lib/test/test_cppext/__init__.py
index 967feee6693c037..5bfc4900cd473c8 100644
--- a/Lib/test/test_cppext/__init__.py
+++ b/Lib/test/test_cppext/__init__.py
@@ -134,6 +134,11 @@ def test_build_cpp14(self):
     def test_build_intel_asm(self):
         self.check_build('_testcppext_asm', extra_cflags='-masm=intel')
 
+    # gh-154643: Test that headers don't use undefined macros in #if
+    @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support -Werror=undef")
+    def test_build_werror_undef(self):
+        self.check_build('_testcppext_undef', extra_cflags='-Werror=undef')
+
 
 class TestInteralCAPI(BaseTests, unittest.TestCase):
     TEST_INTERNAL_C_API = True
diff --git a/Misc/NEWS.d/next/C_API/2026-07-24-22-30-40.gh-issue-154643.z-ZutK.rst b/Misc/NEWS.d/next/C_API/2026-07-24-22-30-40.gh-issue-154643.z-ZutK.rst
new file mode 100644
index 000000000000000..32c26af7e45b7b4
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2026-07-24-22-30-40.gh-issue-154643.z-ZutK.rst
@@ -0,0 +1,2 @@
+In ``pyport.h`` and ``pyatomic.h`` don't use ``__STDC_VERSION__`` when it is
+not defined, fixing build errors for C++ code built with ``-Werror=undef``.