diff --git a/icu4c/source/common/putil.c b/icu4c/source/common/putil.c
index a99f64adc0..8aafb9f37e 100644
--- a/icu4c/source/common/putil.c
+++ b/icu4c/source/common/putil.c
@@ -36,13 +36,16 @@
 ******************************************************************************
 */
 
-#ifndef PTX
-
 /* Define _XOPEN_SOURCE for Solaris and friends. */
 /* NetBSD needs it to be >= 4 */
 #ifndef _XOPEN_SOURCE
+#if __STDC_VERSION__ >= 199901L
+/* It is invalid to compile an XPG3, XPG4, XPG4v2 or XPG5 application using c99 */
+#define _XOPEN_SOURCE 600
+#else
 #define _XOPEN_SOURCE 4
 #endif
+#endif
 
 /* Define __USE_POSIX and __USE_XOPEN for Linux and glibc. */
 #ifndef __USE_POSIX
@@ -52,8 +55,6 @@
 #define __USE_XOPEN
 #endif
 
-#endif /* PTX */
-
 /* include ICU headers */
 #include "unicode/utypes.h"
 #include "unicode/putil.h"