diff --git a/src/decoratedclient.h b/src/decoratedclient.h
index 1b6aa842cd809fe171ce9f0e56e082ebf85b7cf9..61a88cf6b8c0842994fabfb7aff117e57fae56c1 100644
--- a/src/decoratedclient.h
+++ b/src/decoratedclient.h
@@ -169,7 +169,13 @@ class KDECORATIONS2_EXPORT DecoratedClient : public QObject
      */
     Q_PROPERTY(bool applicationMenuActive READ isApplicationMenuActive NOTIFY applicationMenuActiveChanged)
 
-    // TODO: properties for windowId and decorationId?
+    /**
+     * The window id of the DecoratedClient
+     * @since 5.21
+     */
+    Q_PROPERTY(qulonglong windowId READ windowId CONSTANT)
+
+    // TODO: property for decorationId?
 
 public:
     DecoratedClient() = delete;