diff --git a/modules/@angular/router/src/config.ts b/modules/@angular/router/src/config.ts
index b33cca44d9..3ca9625b6d 100644
--- a/modules/@angular/router/src/config.ts
+++ b/modules/@angular/router/src/config.ts
@@ -3,6 +3,10 @@ import {Type} from '@angular/core';
 export type RouterConfig = Route[];
 
 export interface Route {
+  /**
+   * Use `path: ''` instead.
+   * @deprecated
+   */
   index?: boolean;
   path?: string;
   terminal?: boolean;