diff --git a/sys/sys/_visible.h b/sys/sys/_visible.h
index de7542db27b5..747b3b0b6114 100644
--- a/sys/sys/_visible.h
+++ b/sys/sys/_visible.h
@@ -205,7 +205,14 @@
 #define	__POSIX_VISIBLE		202405
 #define	__XSI_VISIBLE		800
 #define	__BSD_VISIBLE		1
-#define	__ISO_C_VISIBLE		2023
+#if _ISOC23_SOURCE || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L)
+#define __ISO_C_VISIBLE		2023
+#elif _ISOC11_SOURCE || (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L)
+#define __ISO_C_VISIBLE		2011
+#else
+/* XXX: Should we permit older? */
+#define	__ISO_C_VISIBLE		1999
+#endif
 #define	__EXT1_VISIBLE		1
 #endif
 #endif /* _POSIX_C_SOURCE */