diff --git a/src/components/block-editor-toolbar/header-toolbar/index.js b/src/components/block-editor-toolbar/header-toolbar/index.js
index 4bdb73e69..76f9ca4cd 100644
--- a/src/components/block-editor-toolbar/header-toolbar/index.js
+++ b/src/components/block-editor-toolbar/header-toolbar/index.js
@@ -26,26 +26,32 @@ function HeaderToolbar( props ) {
 	const inserterButton = useRef();
 	const { setIsInserterOpened } = useDispatch( 'isolated/editor' );
 	const isMobileViewport = useViewportMatch( 'medium', '<' );
-	const { hasFixedToolbar, isInserterEnabled, isTextModeEnabled, previewDeviceType, isInserterOpened } = useSelect(
-		( select ) => {
-			const { hasInserterItems, getBlockRootClientId, getBlockSelectionEnd } = select( 'core/block-editor' );
+	const {
+		hasFixedToolbar,
+		hasPeers,
+		isInserterEnabled,
+		isTextModeEnabled,
+		previewDeviceType,
+		isInserterOpened,
+	} = useSelect( ( select ) => {
+		const { hasInserterItems, getBlockRootClientId, getBlockSelectionEnd } = select( 'core/block-editor' );
 
-			return {
-				hasFixedToolbar: select( 'isolated/editor' ).isFeatureActive( 'fixedToolbar' ),
-				// This setting (richEditingEnabled) should not live in the block editor's setting.
-				isInserterEnabled:
-					select( 'isolated/editor' ).getEditorMode() === 'visual' &&
-					select( 'core/editor' ).getEditorSettings().richEditingEnabled &&
-					hasInserterItems( getBlockRootClientId( getBlockSelectionEnd() ) ),
-				isTextModeEnabled: select( 'isolated/editor' ).getEditorMode() === 'text',
-				previewDeviceType: 'Desktop',
-				isInserterOpened: select( 'isolated/editor' ).isInserterOpened(),
-			};
-		},
-		[]
-	);
+		return {
+			hasFixedToolbar: select( 'isolated/editor' ).isFeatureActive( 'fixedToolbar' ),
+			hasPeers: select( 'isolated/editor' ).hasPeers(),
+			// This setting (richEditingEnabled) should not live in the block editor's setting.
+			isInserterEnabled:
+				select( 'isolated/editor' ).getEditorMode() === 'visual' &&
+				select( 'core/editor' ).getEditorSettings().richEditingEnabled &&
+				hasInserterItems( getBlockRootClientId( getBlockSelectionEnd() ) ),
+			isTextModeEnabled: select( 'isolated/editor' ).getEditorMode() === 'text',
+			previewDeviceType: 'Desktop',
+			isInserterOpened: select( 'isolated/editor' ).isInserterOpened(),
+		};
+	}, [] );
 	const isLargeViewport = useViewportMatch( 'medium' );
-	const { inserter, toc, navigation, undo } = props.settings.iso.toolbar;
+	const { inserter, toc, navigation, undo: undoSetting } = props.settings.iso.toolbar;
+	const undo = undoSetting && ! hasPeers;
 	const displayBlockToolbar = ! isLargeViewport || previewDeviceType !== 'Desktop' || hasFixedToolbar;
 	const toolbarAriaLabel = displayBlockToolbar
 		? /* translators: accessibility text for the editor toolbar when Top Toolbar is on */
diff --git a/src/components/block-editor/index.js b/src/components/block-editor/index.js
index da6ddb8a4..1c0527211 100644
--- a/src/components/block-editor/index.js
+++ b/src/components/block-editor/index.js
@@ -66,11 +66,26 @@ function BlockEditor( props ) {
 	);
 }
 
-export default withDispatch( ( dispatch ) => {
+export default withDispatch( ( dispatch, _ownProps, { select } ) => {
+	const hasPeers = select( 'isolated/editor' ).hasPeers;
 	const { redo, undo } = dispatch( 'isolated/editor' );
 
+	const maybeUndo = ( actionCreator ) => () => {
+		if ( hasPeers() ) {
+			const noticeId = 'isolated/undo-disabled';
+			dispatch( 'core/notices' ).removeNotice( noticeId );
+			return dispatch( 'core/notices' ).createNotice(
+				'warning',
+				'Undo/redo is disabled while editing with other users.',
+				{ id: noticeId }
+			);
+		} else {
+			return actionCreator();
+		}
+	};
+
 	return {
-		redo,
-		undo,
+		redo: maybeUndo( redo ),
+		undo: maybeUndo( undo ),
 	};
 } )( BlockEditor );
diff --git a/src/store/peers/selectors.js b/src/store/peers/selectors.js
index 1de714372..dd9274ae8 100644
--- a/src/store/peers/selectors.js
+++ b/src/store/peers/selectors.js
@@ -1,3 +1,7 @@
 export function getPeers( state ) {
 	return state.peers;
 }
+
+export function hasPeers( state ) {
+	return Object.keys( state.peers ).length > 0;
+}