diff --git a/Include/cpython/pyatomic.h b/Include/cpython/pyatomic.h
index 71a76bdc82ce475..0a11f7781f07e00 100644
--- a/Include/cpython/pyatomic.h
+++ b/Include/cpython/pyatomic.h
@@ -585,7 +585,7 @@ static inline void _Py_atomic_fence_release(void);
 #  define Py_ATOMIC_GCC_H
 #  include "pyatomic_gcc.h"
 #  undef Py_ATOMIC_GCC_H
-#elif _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_ATOMICS__)
+#elif defined(_Py_STDC_VERSION) && _Py_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 3de7fb3d317521a..ee1d8c0e2483dfd 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 _Py_STDC_VERSION >= 201112L && !defined(__STDC_NO_THREADS__)
+#  elif defined(_Py_STDC_VERSION) && _Py_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_cext/__init__.py b/Lib/test/test_cext/__init__.py
index 185f10911370347..4c7c7ef68e0f855 100644
--- a/Lib/test/test_cext/__init__.py
+++ b/Lib/test/test_cext/__init__.py
@@ -133,6 +133,11 @@ def test_build_abi3t(self):
         # Test with Py_TARGET_ABI3T
         self.check_build('_test_cext_abi3t', abi3t=True)
 
+    # 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('_test_cext_undef', extra_cflags='-Werror=undef')
+
 
 class TestPublicCpp(BaseTests, unittest.TestCase):
     LANGUAGE = 'C++'
@@ -174,6 +179,11 @@ def test_build_abi3t(self):
         # Test with Py_TARGET_ABI3T
         self.check_build('_test_cppext_abi3t', abi3t=True)
 
+    # 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 TestInternalC(BaseTests, unittest.TestCase):
     LANGUAGE = 'C'
diff --git a/Lib/test/test_cext/setup.py b/Lib/test/test_cext/setup.py
index 10a3887931932ac..3b613ee729f52c4 100644
--- a/Lib/test/test_cext/setup.py
+++ b/Lib/test/test_cext/setup.py
@@ -42,6 +42,9 @@
 
         # Ask for strict(er) compliance with the standard
         '-pedantic-errors',
+
+        # gh-154643: Headers should not use undefined macros in #if
+        '-Werror=undef',
     ]
     if not support.Py_GIL_DISABLED:
         PUBLIC_CFLAGS.append(
@@ -94,6 +97,9 @@
 
         # We also use `long long`, a C++11 feature we can enable individually.
         '-Wno-long-long',
+
+        # gh-154643: Headers should not use undefined macros in #if
+        '-Werror=undef',
     ]
     INTERNAL_CXXFLAGS = [
         # Do not complain about unused parameteres in the internal C API
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..17c2a58555e7ae9
--- /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 ``_Py_STDC_VERSION`` when it is
+not defined, fixing build errors for C or C++ code built with ``-Werror=undef``.