diff --git a/packages/react-devtools-core/src/standalone.js b/packages/react-devtools-core/src/standalone.js
index c105f20cc8449..7bd636b80e448 100644
--- a/packages/react-devtools-core/src/standalone.js
+++ b/packages/react-devtools-core/src/standalone.js
@@ -86,6 +86,7 @@ function reload() {
         bridge: ((bridge: any): Bridge),
         showTabBar: true,
         store: ((store: any): Store),
+        warnIfLegacyBackendDetected: true,
         viewElementSourceFunction,
         viewElementSourceRequiresFileLocation: true,
       })
diff --git a/shells/dev/src/devtools.js b/shells/dev/src/devtools.js
index 80d8bb38ccb1c..2afed0c208637 100644
--- a/shells/dev/src/devtools.js
+++ b/shells/dev/src/devtools.js
@@ -77,6 +77,7 @@ inject('dist/app.js', () => {
           browserTheme: 'light',
           showTabBar: true,
           store,
+          warnIfLegacyBackendDetected: true,
         })
       );
       batch.then(() => {
diff --git a/src/bridge.js b/src/bridge.js
index c5ed0f4a0379c..23b887af7fc7e 100644
--- a/src/bridge.js
+++ b/src/bridge.js
@@ -128,6 +128,12 @@ export default class Bridge extends EventEmitter<{|
       }) || null;
   }
 
+  // Listening directly to the wall isn't advised.
+  // It can be used to listen for legacy (v3) messages (since they use a different format).
+  get wall(): Wall {
+    return this._wall;
+  }
+
   send(event: string, payload: any, transferable?: Array<any>) {
     if (this._isShutdown) {
       console.warn(
diff --git a/src/devtools/views/DevTools.js b/src/devtools/views/DevTools.js
index 7a04ee46bc831..f8d9bcec9186b 100644
--- a/src/devtools/views/DevTools.js
+++ b/src/devtools/views/DevTools.js
@@ -18,6 +18,7 @@ import ViewElementSourceContext from './Components/ViewElementSourceContext';
 import { ProfilerContextController } from './Profiler/ProfilerContext';
 import { ModalDialogContextController } from './ModalDialog';
 import ReactLogo from './ReactLogo';
+import WarnIfLegacyBackendDetected from './WarnIfLegacyBackendDetected';
 
 import styles from './DevTools.css';
 
@@ -38,6 +39,7 @@ export type Props = {|
   defaultTab?: TabID,
   showTabBar?: boolean,
   store: Store,
+  warnIfLegacyBackendDetected?: boolean,
   viewElementSourceFunction?: ?ViewElementSource,
   viewElementSourceRequiresFileLocation?: boolean,
 
@@ -80,6 +82,7 @@ export default function DevTools({
   settingsPortalContainer,
   showTabBar = false,
   store,
+  warnIfLegacyBackendDetected = false,
   viewElementSourceFunction,
   viewElementSourceRequiresFileLocation = false,
 }: Props) {
@@ -143,6 +146,7 @@ export default function DevTools({
               </TreeContextController>
             </ViewElementSourceContext.Provider>
           </SettingsContextController>
+          {warnIfLegacyBackendDetected && <WarnIfLegacyBackendDetected />}
         </ModalDialogContextController>
       </StoreContext.Provider>
     </BridgeContext.Provider>
diff --git a/src/devtools/views/ModalDialog.js b/src/devtools/views/ModalDialog.js
index b9ca5cc974696..2cceb9a49e034 100644
--- a/src/devtools/views/ModalDialog.js
+++ b/src/devtools/views/ModalDialog.js
@@ -18,6 +18,7 @@ type DIALOG_ACTION_HIDE = {|
 |};
 type DIALOG_ACTION_SHOW = {|
   type: 'SHOW',
+  canBeDismissed?: boolean,
   content: React$Node,
   title?: React$Node | null,
 |};
@@ -27,6 +28,7 @@ type Action = DIALOG_ACTION_HIDE | DIALOG_ACTION_SHOW;
 type Dispatch = (action: Action) => void;
 
 type State = {|
+  canBeDismissed: boolean,
   content: React$Node | null,
   isVisible: boolean,
   title: React$Node | null,
@@ -46,12 +48,14 @@ function dialogReducer(state, action) {
   switch (action.type) {
     case 'HIDE':
       return {
+        canBeDismissed: true,
         content: null,
         isVisible: false,
         title: null,
       };
     case 'SHOW':
       return {
+        canBeDismissed: action.canBeDismissed !== false,
         content: action.content,
         isVisible: true,
         title: action.title || null,
@@ -67,6 +71,7 @@ type Props = {|
 
 function ModalDialogContextController({ children }: Props) {
   const [state, dispatch] = useReducer<State, Action>(dialogReducer, {
+    canBeDismissed: true,
     content: null,
     isVisible: false,
     title: null,
@@ -74,6 +79,7 @@ function ModalDialogContextController({ children }: Props) {
 
   const value = useMemo<ModalDialogContextType>(
     () => ({
+      canBeDismissed: state.canBeDismissed,
       content: state.content,
       isVisible: state.isVisible,
       title: state.title,
@@ -95,10 +101,14 @@ function ModalDialog(_: {||}) {
 }
 
 function ModalDialogImpl(_: {||}) {
-  const { content, dispatch, title } = useContext(ModalDialogContext);
-  const dismissModal = useCallback(() => dispatch({ type: 'HIDE' }), [
-    dispatch,
-  ]);
+  const { canBeDismissed, content, dispatch, title } = useContext(
+    ModalDialogContext
+  );
+  const dismissModal = useCallback(() => {
+    if (canBeDismissed) {
+      dispatch({ type: 'HIDE' });
+    }
+  }, [canBeDismissed, dispatch]);
   const modalRef = useRef<HTMLDivElement | null>(null);
 
   useModalDismissSignal(modalRef, dismissModal);
@@ -108,11 +118,13 @@ function ModalDialogImpl(_: {||}) {
       <div className={styles.Dialog} ref={modalRef}>
         {title !== null && <div className={styles.Title}>{title}</div>}
         {content}
-        <div className={styles.Buttons}>
-          <Button autoFocus onClick={dismissModal}>
-            Okay
-          </Button>
-        </div>
+        {canBeDismissed && (
+          <div className={styles.Buttons}>
+            <Button autoFocus onClick={dismissModal}>
+              Okay
+            </Button>
+          </div>
+        )}
       </div>
     </div>
   );
diff --git a/src/devtools/views/WarnIfLegacyBackendDetected.css b/src/devtools/views/WarnIfLegacyBackendDetected.css
new file mode 100644
index 0000000000000..e1c3817309f6c
--- /dev/null
+++ b/src/devtools/views/WarnIfLegacyBackendDetected.css
@@ -0,0 +1,6 @@
+.Command {
+  background-color: var(--color-dimmest);
+  padding: 0.25rem 0.5rem;
+  display: block;
+  border-radius: 0.125rem;
+}
diff --git a/src/devtools/views/WarnIfLegacyBackendDetected.js b/src/devtools/views/WarnIfLegacyBackendDetected.js
new file mode 100644
index 0000000000000..79dd8978cdcc2
--- /dev/null
+++ b/src/devtools/views/WarnIfLegacyBackendDetected.js
@@ -0,0 +1,60 @@
+// @flow
+
+import React, { Fragment, useContext, useEffect } from 'react';
+import { BridgeContext } from './context';
+import { ModalDialogContext } from './ModalDialog';
+
+import styles from './WarnIfLegacyBackendDetected.css';
+
+export default function WarnIfLegacyBackendDetected(_: {||}) {
+  const bridge = useContext(BridgeContext);
+  const { dispatch } = useContext(ModalDialogContext);
+
+  // Detect pairing with legacy v3 backend.
+  // We do this by listening to a message that it broadcasts but the v4 backend doesn't.
+  // In this case the frontend should show upgrade instructions.
+  useEffect(() => {
+    // Wall.listen returns a cleanup function
+    let unlisten = bridge.wall.listen(event => {
+      switch (event.type) {
+        case 'call':
+        case 'event':
+        case 'many-events':
+          // Any of these types indicate the v3 backend.
+          dispatch({
+            canBeDismissed: false,
+            type: 'SHOW',
+            title:
+              'React DevTools v4 is incompatible with this version of React',
+            content: <InvalidBackendDetected />,
+          });
+
+          if (typeof unlisten === 'function') {
+            unlisten();
+            unlisten = null;
+          }
+          break;
+        default:
+          break;
+      }
+    });
+
+    return () => {
+      if (typeof unlisten === 'function') {
+        unlisten();
+        unlisten = null;
+      }
+    };
+  }, [bridge, dispatch]);
+
+  return null;
+}
+
+function InvalidBackendDetected(_: {||}) {
+  return (
+    <Fragment>
+      <p>Either upgrade React or install React DevTools v3:</p>
+      <code className={styles.Command}>npm install -d react-devtools@^3</code>
+    </Fragment>
+  );
+}