diff --git a/Examples/UIExplorer/js/messagingtest.html b/Examples/UIExplorer/js/messagingtest.html
index c9258dbf5104..61300ec6f789 100644
--- a/Examples/UIExplorer/js/messagingtest.html
+++ b/Examples/UIExplorer/js/messagingtest.html
@@ -15,6 +15,7 @@
   <script>
     var messagesReceivedFromReactNative = 0;
     document.addEventListener('message', function(e) {
+      if (e.origin !== 'react-native') return;
       messagesReceivedFromReactNative += 1;
       document.getElementsByTagName('p')[0].innerHTML =
         'Messages recieved from React Native: ' + messagesReceivedFromReactNative;
@@ -22,7 +23,7 @@
     });
 
     document.getElementsByTagName('button')[0].addEventListener('click', function() {
-      window.postMessage('"Hello" from the web view');
+      window.postMessage('"Hello" from the web view', 'react-native');
     });
   </script>
 </html>
diff --git a/Libraries/Components/WebView/WebView.ios.js b/Libraries/Components/WebView/WebView.ios.js
index 7961d777f7fc..0d6ae00c3de3 100644
--- a/Libraries/Components/WebView/WebView.ios.js
+++ b/Libraries/Components/WebView/WebView.ios.js
@@ -236,13 +236,17 @@ class WebView extends React.Component {
      */
     onNavigationStateChange: PropTypes.func,
     /**
-     * A function that is invoked when the webview calls `window.postMessage`.
-     * Setting this property will inject a `postMessage` global into your
-     * webview, but will still call pre-existing values of `postMessage`.
+     * A function that will be invoked when the webview calls `window.postMessage`.
+     * Setting this property will alter `postMessage` to allow posting to React
+     * Native. `postMessage accepts the arguments `data`, `targetOrigin`, and
+     * `transfer`.
      *
-     * `window.postMessage` accepts one argument, `data`, which will be
-     * available on the event object, `event.nativeEvent.data`. `data`
-     * must be a string.
+     * Calling `postMessage` with a `targetOrigin` of `react-native` will post
+     * to React Native. All other values for `targetOrigin` will use the default
+     * HTML5 behaviour.
+     *
+     * Note that when posting to React Native, `data` must be a string. The data
+     * can be retrieved from this handler with `event.nativeEvent.data`.
      */
     onMessage: PropTypes.func,
     /**
@@ -475,10 +479,15 @@ class WebView extends React.Component {
    * Posts a message to the web view, which will emit a `message` event.
    * Accepts one argument, `data`, which must be a string.
    *
+   * The message event will have the properties `data` and `origin`, where
+   * `data` is the string provided to `postMessage` and `origin` is `react-native`
+   *
    * In your webview, you'll need to something like the following.
    *
    * ```js
-   * document.addEventListener('message', e => { document.title = e.data; });
+   * document.addEventListener('message', e => {
+   *   if (e.origin === 'react-native') doSomethingWith(e.data);
+   * });
    * ```
    */
   postMessage = (data) => {
diff --git a/React/Views/RCTWebView.m b/React/Views/RCTWebView.m
index 23fe37b84dca..64d8d12424c7 100644
--- a/React/Views/RCTWebView.m
+++ b/React/Views/RCTWebView.m
@@ -87,10 +87,15 @@ - (void)stopLoading
 - (void)postMessage:(NSString *)message
 {
   NSDictionary *eventInitDict = @{
+    @"origin": @"react-native",
     @"data": message,
   };
   NSString *source = [NSString
-    stringWithFormat:@"document.dispatchEvent(new MessageEvent('message', %@));",
+    stringWithFormat:
+      @"document.dispatchEvent(new MessageEvent('message', %@));"
+      "if (window.native && typeof window.native.onmessage === 'function') {"
+        "window.native.onmessage(new MessageEvent('message', %@))"
+      "}",
     RCTJSONStringify(eventInitDict, NULL)
   ];
   [_webView stringByEvaluatingJavaScriptFromString:source];
@@ -275,21 +280,20 @@ - (void)webView:(__unused UIWebView *)webView didFailLoadWithError:(NSError *)er
 - (void)webViewDidFinishLoad:(UIWebView *)webView
 {
   if (_messagingEnabled) {
-    #if RCT_DEV
-    // See isNative in lodash
-    NSString *testPostMessageNative = @"String(window.postMessage) === String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage')";
-    BOOL postMessageIsNative = [
-      [webView stringByEvaluatingJavaScriptFromString:testPostMessageNative]
-      isEqualToString:@"true"
-    ];
-    if (!postMessageIsNative) {
-      RCTLogError(@"Setting onMessage on a WebView overrides existing values of window.postMessage, but a previous value was defined");
-    }
-    #endif
     NSString *source = [NSString stringWithFormat:
-      @"window.originalPostMessage = window.postMessage;"
-      "window.postMessage = function(data) {"
-        "window.location = '%@://%@?' + encodeURIComponent(String(data));"
+      @"window.native = {"
+        "postMessage: function(data) {"
+          "window.location = '%@://%@?' + encodeURIComponent(String(data));"
+        "},"
+        "onmessage: null"
+      "};"
+      "window.originalPostMessage = window.postMessage;"
+      "window.postMessage = function(data, targetOrigin) {"
+        "if (targetOrigin === 'react-native') {"
+          "window.location = '%@://%@?' + encodeURIComponent(String(data));"
+        "} else {"
+          "window.originalPostMessage.apply(arguments);"
+        "}"
       "};", RCTJSNavigationScheme, RCTJSPostMessageHost
     ];
     [webView stringByEvaluatingJavaScriptFromString:source];
diff --git a/ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java b/ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java
index 1a80722fe00b..b33e84ea5581 100644
--- a/ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java
+++ b/ReactAndroid/src/main/java/com/facebook/react/views/webview/ReactWebViewManager.java
@@ -273,23 +273,20 @@ public void callInjectedJavaScript() {
 
     public void linkBridge() {
       if (messagingEnabled) {
-        if (ReactBuildConfig.DEBUG && Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
-          // See isNative in lodash
-          String testPostMessageNative = "String(window.postMessage) === String(Object.hasOwnProperty).replace('hasOwnProperty', 'postMessage')";
-          evaluateJavascript(testPostMessageNative, new ValueCallback<String>() {
-            @Override
-            public void onReceiveValue(String value) {
-              if (value.equals("true")) {
-                FLog.w(ReactConstants.TAG, "Setting onMessage on a WebView overrides existing values of window.postMessage, but a previous value was defined");
-              }
-            }
-          });
-        }
-
         loadUrl("javascript:(" +
+          "window.native = {" +
+            "postMessage: function(data) {" +
+              BRIDGE_NAME + ".postMessage(String(data));" +
+            "}," +
+            "onmessage: null" +
+          "}," +
           "window.originalPostMessage = window.postMessage," +
-          "window.postMessage = function(data) {" +
-            BRIDGE_NAME + ".postMessage(String(data));" +
+          "window.postMessage = function(data, targetOrigin) {" +
+            "if (targetOrigin === 'react-native') {" +
+              BRIDGE_NAME + ".postMessage(String(data));" +
+            "} else {" +
+              "window.originalPostMessage.apply(arguments);" +
+            "}" +
           "}" +
         ")");
       }
@@ -493,8 +490,13 @@ public void receiveCommand(WebView root, int commandId, @Nullable ReadableArray
       case COMMAND_POST_MESSAGE:
         try {
           JSONObject eventInitDict = new JSONObject();
+          eventInitDict.put("origin", "react-native");
           eventInitDict.put("data", args.getString(0));
-          root.loadUrl("javascript:(document.dispatchEvent(new MessageEvent('message', " + eventInitDict.toString() + ")))");
+          root.loadUrl("javascript:(" +
+            "document.dispatchEvent(new MessageEvent('message', " + eventInitDict.toString() + "))," +
+            "window.native && typeof window.native.onmessage === 'function' &&" +
+              "window.native.onmessage(new MessageEvent('message', " + eventInitDict.toString() + "))" +
+          ")");
         } catch (JSONException e) {
           throw new RuntimeException(e);
         }