diff --git a/include/ChangeLog b/include/ChangeLog
index 9b7a139bc26..52903816ac0 100644
--- a/include/ChangeLog
+++ b/include/ChangeLog
@@ -1,3 +1,7 @@
+2017-05-19  Eli Zaretskii <eliz@gnu.org>
+
+	* environ.h: Add #ifndef guard.
+
 2017-05-18  Martin Liska  <mliska@suse.cz>
 
 	* ansidecl.h: Define CONSTEXPR macro.
diff --git a/include/environ.h b/include/environ.h
index 326a27812db..32bb0b155c9 100644
--- a/include/environ.h
+++ b/include/environ.h
@@ -27,7 +27,9 @@ Boston, MA 02110-1301, USA.  */
 #     include <crt_externs.h>
 #     define environ (*_NSGetEnviron ())
 #  else
+#    ifndef environ
 extern char **environ;
+#    endif
 #  endif
 #  define HAVE_ENVIRON_DECL
 #endif