diff --git a/src/PythonQtPythonInclude.h b/src/PythonQtPythonInclude.h
index 673db874f..90569ddd5 100644
--- a/src/PythonQtPythonInclude.h
+++ b/src/PythonQtPythonInclude.h
@@ -142,4 +142,19 @@
 #define PyBytes_FromStringAndSize PyString_FromStringAndSize
 #endif
 
+// Avoid clashes with libstdc++ <locale> by undefining ctype macros
+// that CPython may introduce on macOS when the UTF-8 ctype quirk is enabled.
+// (_PY_PORT_CTYPE_UTF8_ISSUE is defined by CPython’s pyport.h; we apply these
+// undefs only in C++ builds.)
+#if defined(_PY_PORT_CTYPE_UTF8_ISSUE) && defined(__cplusplus)
+#undef isalnum
+#undef isalpha
+#undef islower
+#undef isspace
+#undef isupper
+#undef tolower
+#undef toupper
 #endif
+
+#endif
+