diff --git a/lib/libc/string/strstr.3 b/lib/libc/string/strstr.3
index 25af1ac6c08..7ff9ec8c54e 100644
--- a/lib/libc/string/strstr.3
+++ b/lib/libc/string/strstr.3
@@ -152,3 +152,9 @@ The
 function
 conforms to
 .St -isoC .
+.Sh HISTORY
+The
+.Fn strnstr
+function was introduced by
+.Fx 4.5
+and is non-standard.