diff --git a/Misc/NEWS.d/next/Build/2026-08-07-00-00-00.gh-issue-155321.pythonh-posix.rst b/Misc/NEWS.d/next/Build/2026-08-07-00-00-00.gh-issue-155321.pythonh-posix.rst
new file mode 100644
index 00000000000000..61b98ee2b297db
--- /dev/null
+++ b/Misc/NEWS.d/next/Build/2026-08-07-00-00-00.gh-issue-155321.pythonh-posix.rst
@@ -0,0 +1,2 @@
+Allow :file:`Python.h` to be included after system headers that define
+``_POSIX_C_SOURCE`` without causing a macro redefinition diagnostic.
diff --git a/configure b/configure
index 836eb88e4cdb6e..1a5b4b59fadb8a 100755
--- a/configure
+++ b/configure
@@ -4809,7 +4809,7 @@ printf "%s\n" "#define _XOPEN_SOURCE_EXTENDED 1" >>confdefs.h
 
 
 
-printf "%s\n" "#define _POSIX_C_SOURCE 202405L" >>confdefs.h
+printf "%s\n" "#define _PYTHON_POSIX_C_SOURCE 1" >>confdefs.h
 
 
   # Defining _POSIX_C_SOURCE and _XOPEN_SOURCE hides C23 library
diff --git a/configure.ac b/configure.ac
index 3a9841b1e7d074..f113a3fd2c7b57 100644
--- a/configure.ac
+++ b/configure.ac
@@ -237,6 +237,15 @@ AH_BOTTOM([
 #define STRICT_SYSV_CURSES /* Don't use ncurses extensions */
 #endif
 
+/* Define _POSIX_C_SOURCE only when it was not defined by an earlier
+   system-header include. */
+#ifdef _PYTHON_POSIX_C_SOURCE
+#  ifndef _POSIX_C_SOURCE
+#    define _POSIX_C_SOURCE 202405L
+#  endif
+#  undef _PYTHON_POSIX_C_SOURCE
+#endif
+
 #endif /*Py_PYCONFIG_H*/
 ])
 
@@ -930,8 +939,8 @@ then
   AC_DEFINE([_XOPEN_SOURCE_EXTENDED], [1],
             [Define to activate Unix95-and-earlier features])
 
-  AC_DEFINE([_POSIX_C_SOURCE], [202405L],
-            [Define to activate features from IEEE Std 1003.1-2024])
+  AC_DEFINE([_PYTHON_POSIX_C_SOURCE], [1],
+            [Define to configure _POSIX_C_SOURCE in pyconfig.h])
 
   # Defining _POSIX_C_SOURCE and _XOPEN_SOURCE hides C23 library
   # declarations on FreeBSD (e.g. sinpi() in math.h) when compiling
diff --git a/pyconfig.h.in b/pyconfig.h.in
index 691c6c0d9feb6d..159a9b73075a5c 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -2161,15 +2161,15 @@
 /* Define on NetBSD to activate all library features */
 #undef _NETBSD_SOURCE
 
-/* Define to activate features from IEEE Std 1003.1-2024 */
-#undef _POSIX_C_SOURCE
-
 /* Define if you have POSIX threads, and your system does not define that. */
 #undef _POSIX_THREADS
 
 /* framework name */
 #undef _PYTHONFRAMEWORK
 
+/* Define to configure _POSIX_C_SOURCE in pyconfig.h */
+#undef _PYTHON_POSIX_C_SOURCE
+
 /* Maximum length in bytes of a thread name */
 #undef _PYTHREAD_NAME_MAXLEN
 
@@ -2263,5 +2263,14 @@
 #define STRICT_SYSV_CURSES /* Don't use ncurses extensions */
 #endif
 
+/* Define _POSIX_C_SOURCE only when it was not defined by an earlier
+   system-header include. */
+#ifdef _PYTHON_POSIX_C_SOURCE
+#  ifndef _POSIX_C_SOURCE
+#    define _POSIX_C_SOURCE 202405L
+#  endif
+#  undef _PYTHON_POSIX_C_SOURCE
+#endif
+
 #endif /*Py_PYCONFIG_H*/