diff --git a/lisp/mouse.el b/lisp/mouse.el
index f076e90bd9..fd0ef2a360 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -1999,6 +1999,8 @@ mouse-drag-region-rectangle
         (unless dragged
           ;; This is actually a drag.
           (setq dragged t)
+	  (unless transient-mark-mode
+	    (setq transient-mark-mode 'lambda))
           (mouse-minibuffer-check start-event)
           (deactivate-mark)
           (posn-set-point start-pos)