diff --git a/src/plugins/platforms/cocoa/qcocoawindow.h b/src/plugins/platforms/cocoa/qcocoawindow.h
index 94d166b7a5..184422a04a 100644
--- a/src/plugins/platforms/cocoa/qcocoawindow.h
+++ b/src/plugins/platforms/cocoa/qcocoawindow.h
@@ -99,6 +99,7 @@ public:
     void raise();
     void lower();
     void propagateSizeHints();
+    void setOpacity(qreal level);
     bool setKeyboardGrabEnabled(bool grab);
     bool setMouseGrabEnabled(bool grab);
 
diff --git a/src/plugins/platforms/cocoa/qcocoawindow.mm b/src/plugins/platforms/cocoa/qcocoawindow.mm
index b5e8ff2246..7282a8d776 100644
--- a/src/plugins/platforms/cocoa/qcocoawindow.mm
+++ b/src/plugins/platforms/cocoa/qcocoawindow.mm
@@ -233,6 +233,12 @@ void QCocoaWindow::propagateSizeHints()
     }
 }
 
+void QCocoaWindow::setOpacity(qreal level)
+{
+    if (m_nsWindow)
+        [m_nsWindow setAlphaValue:level];
+}
+
 bool QCocoaWindow::setKeyboardGrabEnabled(bool grab)
 {
     if (!m_nsWindow)