diff --git a/gdk/x11/gdkwindow-x11.h b/gdk/x11/gdkwindow-x11.h
index fd63e8b462..0c02b67652 100644
--- a/gdk/x11/gdkwindow-x11.h
+++ b/gdk/x11/gdkwindow-x11.h
@@ -35,6 +35,7 @@
 #endif
 
 #ifdef HAVE_XSYNC
+#include <X11/Xlib.h>
 #include <X11/extensions/sync.h>
 #endif