diff --git a/openbox/prop.c b/openbox/prop.c
index b3349e32..dca04344 100644
--- a/openbox/prop.c
+++ b/openbox/prop.c
@@ -51,6 +51,8 @@ void prop_startup()
     CREATE(net_wm_moveresize, "_NET_WM_MOVERESIZE");
     CREATE(net_moveresize_window, "_NET_MOVERESIZE_WINDOW");
 
+    CREATE(net_startup_id, "_NET_STARTUP_ID");
+
     CREATE(net_wm_name, "_NET_WM_NAME");
     CREATE(net_wm_visible_name, "_NET_WM_VISIBLE_NAME");
     CREATE(net_wm_icon_name, "_NET_WM_ICON_NAME");
diff --git a/openbox/prop.h b/openbox/prop.h
index 5bfad572..a9d1b573 100644
--- a/openbox/prop.h
+++ b/openbox/prop.h
@@ -56,6 +56,10 @@ typedef struct Atoms {
     Atom net_close_window;
     Atom net_wm_moveresize;
     Atom net_moveresize_window;
+
+    /* startup-notification extension */
+    Atom net_startup_id;
+
     /* application window properties */
     Atom net_wm_name;
     Atom net_wm_visible_name;