diff --git a/packages/react-devtools-shared/src/__tests__/__snapshots__/profilingCache-test.js.snap b/packages/react-devtools-shared/src/__tests__/__snapshots__/profilingCache-test.js.snap
index f07f405a85f2e..dd4717bb5cb6b 100644
--- a/packages/react-devtools-shared/src/__tests__/__snapshots__/profilingCache-test.js.snap
+++ b/packages/react-devtools-shared/src/__tests__/__snapshots__/profilingCache-test.js.snap
@@ -719,6 +719,8 @@ Object {
           1,
           1,
           11,
+          0,
+          1,
           1,
           1,
           4,
@@ -1183,6 +1185,8 @@ Object {
           1,
           1,
           11,
+          0,
+          1,
           1,
           1,
           4,
@@ -1658,6 +1662,8 @@ Object {
       1,
       13,
       11,
+      0,
+      1,
       1,
       1,
       4,
@@ -2202,6 +2208,8 @@ Object {
           1,
           13,
           11,
+          0,
+          1,
           1,
           1,
           4,
@@ -2295,6 +2303,8 @@ Object {
       1,
       1,
       11,
+      0,
+      1,
       1,
       1,
       1,
@@ -2943,6 +2953,8 @@ Object {
           1,
           1,
           11,
+          0,
+          1,
           1,
           1,
           1,
@@ -4214,6 +4226,8 @@ Object {
           1,
           1,
           11,
+          0,
+          1,
           1,
           1,
           1,
diff --git a/packages/react-devtools-shared/src/backend/legacy/renderer.js b/packages/react-devtools-shared/src/backend/legacy/renderer.js
index b757d008ac294..0a1896ee322c4 100644
--- a/packages/react-devtools-shared/src/backend/legacy/renderer.js
+++ b/packages/react-devtools-shared/src/backend/legacy/renderer.js
@@ -386,7 +386,9 @@ export function attach(
       pushOperation(TREE_OPERATION_ADD);
       pushOperation(id);
       pushOperation(ElementTypeRoot);
-      pushOperation(0); // isProfilingSupported?
+      pushOperation(0); // StrictMode compliant?
+      pushOperation(0); // Profiling supported?
+      pushOperation(0); // StrictMode supported?
       pushOperation(hasOwnerMetadata ? 1 : 0);
     } else {
       const type = getElementType(internalInstance);
diff --git a/packages/react-devtools-shared/src/backend/renderer.js b/packages/react-devtools-shared/src/backend/renderer.js
index 7aebe6e507a0f..631521791a549 100644
--- a/packages/react-devtools-shared/src/backend/renderer.js
+++ b/packages/react-devtools-shared/src/backend/renderer.js
@@ -24,6 +24,7 @@ import {
   ElementTypeRoot,
   ElementTypeSuspense,
   ElementTypeSuspenseList,
+  StrictMode,
 } from 'react-devtools-shared/src/types';
 import {
   deletePathInObject,
@@ -52,6 +53,7 @@ import {
   TREE_OPERATION_REMOVE,
   TREE_OPERATION_REMOVE_ROOT,
   TREE_OPERATION_REORDER_CHILDREN,
+  TREE_OPERATION_SET_SUBTREE_MODE,
   TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS,
   TREE_OPERATION_UPDATE_TREE_BASE_DURATION,
 } from '../constants';
@@ -155,6 +157,7 @@ export function getInternalReactConstants(
   ReactPriorityLevels: ReactPriorityLevelsType,
   ReactTypeOfSideEffect: ReactTypeOfSideEffectType,
   ReactTypeOfWork: WorkTagMap,
+  StrictModeBits: number,
 |} {
   const ReactTypeOfSideEffect: ReactTypeOfSideEffectType = {
     DidCapture: 0b10000000,
@@ -192,6 +195,18 @@ export function getInternalReactConstants(
     };
   }
 
+  let StrictModeBits = 0;
+  if (gte(version, '18.0.0-alpha')) {
+    // 18+
+    StrictModeBits = 0b011000;
+  } else if (gte(version, '16.9.0')) {
+    // 16.9 - 17
+    StrictModeBits = 0b1;
+  } else if (gte(version, '16.3.0')) {
+    // 16.3 - 16.8
+    StrictModeBits = 0b10;
+  }
+
   let ReactTypeOfWork: WorkTagMap = ((null: any): WorkTagMap);
 
   // **********************************************************
@@ -513,6 +528,7 @@ export function getInternalReactConstants(
     ReactPriorityLevels,
     ReactTypeOfWork,
     ReactTypeOfSideEffect,
+    StrictModeBits,
   };
 }
 
@@ -534,6 +550,7 @@ export function attach(
     ReactPriorityLevels,
     ReactTypeOfWork,
     ReactTypeOfSideEffect,
+    StrictModeBits,
   } = getInternalReactConstants(version);
   const {
     DidCapture,
@@ -1876,7 +1893,9 @@ export function attach(
       pushOperation(TREE_OPERATION_ADD);
       pushOperation(id);
       pushOperation(ElementTypeRoot);
+      pushOperation((fiber.mode & StrictModeBits) !== 0 ? 1 : 0);
       pushOperation(isProfilingSupported ? 1 : 0);
+      pushOperation(StrictModeBits !== 0 ? 1 : 0);
       pushOperation(hasOwnerMetadata ? 1 : 0);
 
       if (isProfiling) {
@@ -1913,6 +1932,16 @@ export function attach(
       pushOperation(ownerID);
       pushOperation(displayNameStringID);
       pushOperation(keyStringID);
+
+      // If this subtree has a new mode, let the frontend know.
+      if (
+        (fiber.mode & StrictModeBits) !== 0 &&
+        (((parentFiber: any): Fiber).mode & StrictModeBits) === 0
+      ) {
+        pushOperation(TREE_OPERATION_SET_SUBTREE_MODE);
+        pushOperation(id);
+        pushOperation(StrictMode);
+      }
     }
 
     if (isProfilingSupported) {
diff --git a/packages/react-devtools-shared/src/bridge.js b/packages/react-devtools-shared/src/bridge.js
index e67f6cee95423..68891e8c18d4b 100644
--- a/packages/react-devtools-shared/src/bridge.js
+++ b/packages/react-devtools-shared/src/bridge.js
@@ -57,6 +57,12 @@ export const BRIDGE_PROTOCOL: Array<BridgeProtocol> = [
   {
     version: 1,
     minNpmVersion: '4.13.0',
+    maxNpmVersion: '4.21.0',
+  },
+  // Version 2 adds a StrictMode-enabled and supports-StrictMode bits to add-root operation.
+  {
+    version: 2,
+    minNpmVersion: '4.22.0',
     maxNpmVersion: null,
   },
 ];
diff --git a/packages/react-devtools-shared/src/constants.js b/packages/react-devtools-shared/src/constants.js
index 8898c27787f40..6480222e1e414 100644
--- a/packages/react-devtools-shared/src/constants.js
+++ b/packages/react-devtools-shared/src/constants.js
@@ -23,6 +23,7 @@ export const TREE_OPERATION_REORDER_CHILDREN = 3;
 export const TREE_OPERATION_UPDATE_TREE_BASE_DURATION = 4;
 export const TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS = 5;
 export const TREE_OPERATION_REMOVE_ROOT = 6;
+export const TREE_OPERATION_SET_SUBTREE_MODE = 7;
 
 export const LOCAL_STORAGE_DEFAULT_TAB_KEY = 'React::DevTools::defaultTab';
 
diff --git a/packages/react-devtools-shared/src/devtools/store.js b/packages/react-devtools-shared/src/devtools/store.js
index c8b252072dcb4..b68ea3d26f92e 100644
--- a/packages/react-devtools-shared/src/devtools/store.js
+++ b/packages/react-devtools-shared/src/devtools/store.js
@@ -14,6 +14,7 @@ import {
   TREE_OPERATION_REMOVE,
   TREE_OPERATION_REMOVE_ROOT,
   TREE_OPERATION_REORDER_CHILDREN,
+  TREE_OPERATION_SET_SUBTREE_MODE,
   TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS,
   TREE_OPERATION_UPDATE_TREE_BASE_DURATION,
 } from '../constants';
@@ -33,6 +34,7 @@ import {
   BRIDGE_PROTOCOL,
   currentBridgeProtocol,
 } from 'react-devtools-shared/src/bridge';
+import {StrictMode} from 'react-devtools-shared/src/types';
 
 import type {Element} from './views/Components/types';
 import type {ComponentFilter, ElementType} from '../types';
@@ -72,6 +74,7 @@ type Config = {|
 export type Capabilities = {|
   hasOwnerMetadata: boolean,
   supportsProfiling: boolean,
+  supportsStrictMode: boolean,
 |};
 
 /**
@@ -812,6 +815,20 @@ export default class Store extends EventEmitter<{|
     }
   };
 
+  _recursivelyUpdateSubtree(
+    id: number,
+    callback: (element: Element) => void,
+  ): void {
+    const element = this._idToElement.get(id);
+    if (element) {
+      callback(element);
+
+      element.children.forEach(child =>
+        this._recursivelyUpdateSubtree(child, callback),
+      );
+    }
+  }
+
   onBridgeNativeStyleEditorSupported = ({
     isSupported,
     validAttributes,
@@ -883,9 +900,15 @@ export default class Store extends EventEmitter<{|
               debug('Add', `new root node ${id}`);
             }
 
+            const isStrictModeCompliant = operations[i] > 0;
+            i++;
+
             const supportsProfiling = operations[i] > 0;
             i++;
 
+            const supportsStrictMode = operations[i] > 0;
+            i++;
+
             const hasOwnerMetadata = operations[i] > 0;
             i++;
 
@@ -894,8 +917,14 @@ export default class Store extends EventEmitter<{|
             this._rootIDToCapabilities.set(id, {
               hasOwnerMetadata,
               supportsProfiling,
+              supportsStrictMode,
             });
 
+            // Not all roots support StrictMode;
+            // don't flag a root as non-compliant unless it also supports StrictMode.
+            const isStrictModeNonCompliant =
+              !isStrictModeCompliant && supportsStrictMode;
+
             this._idToElement.set(id, {
               children: [],
               depth: -1,
@@ -903,6 +932,7 @@ export default class Store extends EventEmitter<{|
               hocDisplayNames: null,
               id,
               isCollapsed: false, // Never collapse roots; it would hide the entire tree.
+              isStrictModeNonCompliant,
               key: null,
               ownerID: 0,
               parentID: 0,
@@ -958,9 +988,10 @@ export default class Store extends EventEmitter<{|
               hocDisplayNames,
               id,
               isCollapsed: this._collapseNodesByDefault,
+              isStrictModeNonCompliant: parentElement.isStrictModeNonCompliant,
               key,
               ownerID,
-              parentID: parentElement.id,
+              parentID,
               type,
               weight: 1,
             };
@@ -1050,6 +1081,7 @@ export default class Store extends EventEmitter<{|
               haveErrorsOrWarningsChanged = true;
             }
           }
+
           break;
         }
         case TREE_OPERATION_REMOVE_ROOT: {
@@ -1124,6 +1156,28 @@ export default class Store extends EventEmitter<{|
           }
           break;
         }
+        case TREE_OPERATION_SET_SUBTREE_MODE: {
+          const id = operations[i + 1];
+          const mode = operations[i + 2];
+
+          i += 3;
+
+          // If elements have already been mounted in this subtree, update them.
+          // (In practice, this likely only applies to the root element.)
+          if (mode === StrictMode) {
+            this._recursivelyUpdateSubtree(id, element => {
+              element.isStrictModeNonCompliant = false;
+            });
+          }
+
+          if (__DEBUG__) {
+            debug(
+              'Subtree mode',
+              `Subtree with root ${id} set to mode ${mode}`,
+            );
+          }
+          break;
+        }
         case TREE_OPERATION_UPDATE_TREE_BASE_DURATION:
           // Base duration updates are only sent while profiling is in progress.
           // We can ignore them at this point.
diff --git a/packages/react-devtools-shared/src/devtools/views/ButtonIcon.js b/packages/react-devtools-shared/src/devtools/views/ButtonIcon.js
index b384018671d92..a5478dbd6dc0d 100644
--- a/packages/react-devtools-shared/src/devtools/views/ButtonIcon.js
+++ b/packages/react-devtools-shared/src/devtools/views/ButtonIcon.js
@@ -35,6 +35,7 @@ export type IconType =
   | 'search'
   | 'settings'
   | 'error'
+  | 'strict-mode-non-compliant'
   | 'suspend'
   | 'undo'
   | 'up'
@@ -121,6 +122,9 @@ export default function ButtonIcon({className = '', type}: Props) {
     case 'error':
       pathData = PATH_ERROR;
       break;
+    case 'strict-mode-non-compliant':
+      pathData = PATH_STRICT_MODE_NON_COMPLIANT;
+      break;
     case 'suspend':
       pathData = PATH_SUSPEND;
       break;
@@ -269,6 +273,10 @@ const PATH_VIEW_DOM = `
   3-1.34 3-3-1.34-3-3-3z
 `;
 
+const PATH_STRICT_MODE_NON_COMPLIANT = `
+  M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm1 15h-2v-2h2v2zm0-4h-2V7h2v6z
+`;
+
 const PATH_VIEW_SOURCE = `
   M9.4 16.6L4.8 12l4.6-4.6L8 6l-6 6 6 6 1.4-1.4zm5.2 0l4.6-4.6-4.6-4.6L16 6l6 6-6 6-1.4-1.4z
   `;
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Element.css b/packages/react-devtools-shared/src/devtools/views/Components/Element.css
index a21b303a3504a..1fa165c32cc08 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Element.css
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Element.css
@@ -43,6 +43,15 @@
   --color-expand-collapse-toggle: var(--color-component-name-inverted);
 }
 
+.SelectedElement.StrictModeNonCompliantElement {
+  background-color: var(--color-warning-background);
+  color: var(--color-text-selected);
+}
+.InactiveSelectedElement.StrictModeNonCompliantElement {
+  background-color: var(--color-error-background);
+  color: var(--color-text-selected);
+}
+
 .KeyName {
   color: var(--color-attribute-name);
 }
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/Element.js b/packages/react-devtools-shared/src/devtools/views/Components/Element.js
index 73789a9bd62ff..ee5a20b5aaaf0 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/Element.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/Element.js
@@ -113,6 +113,7 @@ export default function Element({data, index, style}: Props) {
     depth,
     displayName,
     hocDisplayNames,
+    isStrictModeNonCompliant,
     key,
     type,
   } = ((element: any): ElementType);
@@ -126,6 +127,10 @@ export default function Element({data, index, style}: Props) {
     className = styles.HoveredElement;
   }
 
+  if (isStrictModeNonCompliant) {
+    className += ' ' + styles.StrictModeNonCompliantElement;
+  }
+
   return (
     <div
       className={className}
@@ -146,6 +151,7 @@ export default function Element({data, index, style}: Props) {
         {ownerID === null ? (
           <ExpandCollapseToggle element={element} store={store} />
         ) : null}
+
         <DisplayName displayName={displayName} id={((id: any): number)} />
         {key && (
           <Fragment>
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.css b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.css
index b6ad3b9983f22..6cd4b86a868b5 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.css
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.css
@@ -66,3 +66,8 @@
   font-style: italic;
   border-left: 1px solid var(--color-border);
 }
+
+.StrictModeNonCompliant {
+  margin-right: 0.125rem;
+  color: var(--color-warning-background);
+}
\ No newline at end of file
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.js b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.js
index 9ebd30a6ec0b3..62a9e4a8d7cb3 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/InspectedElement.js
@@ -235,9 +235,22 @@ export default function InspectedElementWrapper(_: Props) {
     );
   }
 
+  let strictModeBadge = null;
+  if (element.isStrictModeNonCompliant) {
+    strictModeBadge = (
+      <Button
+        className={styles.StrictModeNonCompliant}
+        title="This component does not run in StrictMode. Click to learn more.">
+        <ButtonIcon type="strict-mode-non-compliant" />
+      </Button>
+    );
+  }
+
   return (
     <div className={styles.InspectedElement}>
       <div className={styles.TitleRow}>
+        {strictModeBadge}
+
         {element.key && (
           <>
             <div className={styles.Key} title={`key "${element.key}"`}>
@@ -248,7 +261,13 @@ export default function InspectedElementWrapper(_: Props) {
         )}
 
         <div className={styles.SelectedComponentName}>
-          <div className={styles.Component} title={element.displayName}>
+          <div
+            className={
+              element.isStrictModeNonCompliant
+                ? styles.StrictModeNonCompliantComponent
+                : styles.Component
+            }
+            title={element.displayName}>
             {element.displayName}
           </div>
         </div>
diff --git a/packages/react-devtools-shared/src/devtools/views/Components/types.js b/packages/react-devtools-shared/src/devtools/views/Components/types.js
index 372cdd5b9087d..c6789fa1e9ec1 100644
--- a/packages/react-devtools-shared/src/devtools/views/Components/types.js
+++ b/packages/react-devtools-shared/src/devtools/views/Components/types.js
@@ -42,6 +42,10 @@ export type Element = {|
   // This property is used to quickly determine the total number of Elements,
   // and the Element at any given index (for windowing purposes).
   weight: number,
+
+  // This element is not in a StrictMode compliant subtree.
+  // Only true for React versions supporting StrictMode.
+  isStrictModeNonCompliant: boolean,
 |};
 
 export type SerializedElement = {|
diff --git a/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js b/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
index 1d16aba7fc292..286048a172bf5 100644
--- a/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
+++ b/packages/react-devtools-shared/src/devtools/views/Profiler/CommitTreeBuilder.js
@@ -13,6 +13,7 @@ import {
   TREE_OPERATION_REMOVE,
   TREE_OPERATION_REMOVE_ROOT,
   TREE_OPERATION_REORDER_CHILDREN,
+  TREE_OPERATION_SET_SUBTREE_MODE,
   TREE_OPERATION_UPDATE_TREE_BASE_DURATION,
   TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS,
 } from 'react-devtools-shared/src/constants';
@@ -179,7 +180,7 @@ function updateTree(
     const operation = operations[i];
 
     switch (operation) {
-      case TREE_OPERATION_ADD:
+      case TREE_OPERATION_ADD: {
         id = ((operations[i + 1]: any): number);
         const type = ((operations[i + 2]: any): ElementType);
 
@@ -192,7 +193,9 @@ function updateTree(
         }
 
         if (type === ElementTypeRoot) {
+          i++; // isStrictModeCompliant
           i++; // supportsProfiling flag
+          i++; // supportsStrictMode flag
           i++; // hasOwnerMetadata flag
 
           if (__DEBUG__) {
@@ -250,6 +253,7 @@ function updateTree(
         }
 
         break;
+      }
       case TREE_OPERATION_REMOVE: {
         const removeLength = ((operations[i + 1]: any): number);
         i += 2;
@@ -307,6 +311,17 @@ function updateTree(
 
         break;
       }
+      case TREE_OPERATION_SET_SUBTREE_MODE: {
+        id = operations[i + 1];
+        const mode = operations[i + 1];
+
+        i += 3;
+
+        if (__DEBUG__) {
+          debug('Subtree mode', `Subtree with root ${id} set to mode ${mode}`);
+        }
+        break;
+      }
       case TREE_OPERATION_UPDATE_TREE_BASE_DURATION: {
         id = operations[i + 1];
 
@@ -323,7 +338,7 @@ function updateTree(
         i += 3;
         break;
       }
-      case TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS:
+      case TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS: {
         id = operations[i + 1];
         const numErrors = operations[i + 2];
         const numWarnings = operations[i + 3];
@@ -337,6 +352,7 @@ function updateTree(
           );
         }
         break;
+      }
 
       default:
         throw Error(`Unsupported Bridge operation "${operation}"`);
diff --git a/packages/react-devtools-shared/src/types.js b/packages/react-devtools-shared/src/types.js
index 5a54979b63b19..859fb9bd8ff72 100644
--- a/packages/react-devtools-shared/src/types.js
+++ b/packages/react-devtools-shared/src/types.js
@@ -100,3 +100,5 @@ export type StyleXPlugin = {|
 export type Plugins = {|
   stylex: StyleXPlugin | null,
 |};
+
+export const StrictMode = 1;
diff --git a/packages/react-devtools-shared/src/utils.js b/packages/react-devtools-shared/src/utils.js
index 0c50374ae30b6..c022cd4650ed7 100644
--- a/packages/react-devtools-shared/src/utils.js
+++ b/packages/react-devtools-shared/src/utils.js
@@ -28,6 +28,7 @@ import {
   TREE_OPERATION_REMOVE,
   TREE_OPERATION_REMOVE_ROOT,
   TREE_OPERATION_REORDER_CHILDREN,
+  TREE_OPERATION_SET_SUBTREE_MODE,
   TREE_OPERATION_UPDATE_ERRORS_OR_WARNINGS,
   TREE_OPERATION_UPDATE_TREE_BASE_DURATION,
 } from './constants';
@@ -211,7 +212,9 @@ export function printOperationsArray(operations: Array<number>) {
         if (type === ElementTypeRoot) {
           logs.push(`Add new root node ${id}`);
 
+          i++; // isStrictModeCompliant
           i++; // supportsProfiling
+          i++; // supportsStrictMode
           i++; // hasOwnerMetadata
         } else {
           const parentID = ((operations[i]: any): number);
@@ -249,6 +252,15 @@ export function printOperationsArray(operations: Array<number>) {
         logs.push(`Remove root ${rootID}`);
         break;
       }
+      case TREE_OPERATION_SET_SUBTREE_MODE: {
+        const id = operations[i + 1];
+        const mode = operations[i + 1];
+
+        i += 3;
+
+        logs.push(`Mode ${mode} set for subtree with root ${id}`);
+        break;
+      }
       case TREE_OPERATION_REORDER_CHILDREN: {
         const id = ((operations[i + 1]: any): number);
         const numChildren = ((operations[i + 2]: any): number);
diff --git a/packages/react-devtools-shell/src/app/PartiallyStrictApp/index.js b/packages/react-devtools-shell/src/app/PartiallyStrictApp/index.js
new file mode 100644
index 0000000000000..068e11935bce6
--- /dev/null
+++ b/packages/react-devtools-shell/src/app/PartiallyStrictApp/index.js
@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import * as React from 'react';
+import {StrictMode} from 'react';
+
+export default function PartiallyStrictApp() {
+  return (
+    <>
+      <Child />
+      <StrictMode>
+        <StrictChild />
+      </StrictMode>
+    </>
+  );
+}
+
+function Child() {
+  return <Grandchild />;
+}
+
+function StrictChild() {
+  return <Grandchild />;
+}
+
+function Grandchild() {
+  return null;
+}
diff --git a/packages/react-devtools-shell/src/app/index.js b/packages/react-devtools-shell/src/app/index.js
index 805cd0a2a707e..6d49d9d871f61 100644
--- a/packages/react-devtools-shell/src/app/index.js
+++ b/packages/react-devtools-shell/src/app/index.js
@@ -6,6 +6,8 @@ import {createElement} from 'react';
 import {
   // $FlowFixMe Flow does not yet know about createRoot()
   createRoot,
+  render,
+  unmountComponentAtNode,
 } from 'react-dom';
 import DeeplyNestedComponents from './DeeplyNestedComponents';
 import Iframe from './Iframe';
@@ -18,6 +20,7 @@ import ReactNativeWeb from './ReactNativeWeb';
 import ToDoList from './ToDoList';
 import Toggle from './Toggle';
 import ErrorBoundaries from './ErrorBoundaries';
+import PartiallyStrictApp from './PartiallyStrictApp';
 import SuspenseTree from './SuspenseTree';
 import {ignoreErrors, ignoreLogs, ignoreWarnings} from './console';
 
@@ -34,20 +37,34 @@ ignoreErrors([
 ignoreWarnings(['Warning: componentWillReceiveProps has been renamed']);
 ignoreLogs([]);
 
-const roots = [];
+const unmountFunctions = [];
 
-function mountHelper(App) {
+function mountHelper(App, useLegacyRender = false) {
   const container = document.createElement('div');
 
   ((document.body: any): HTMLBodyElement).appendChild(container);
 
-  const root = createRoot(container);
-  root.render(createElement(App));
+  if (useLegacyRender) {
+    render(createElement(App), container);
 
-  roots.push(root);
+    unmountFunctions.push(() => unmountComponentAtNode(container));
+  } else {
+    const root = createRoot(container);
+    root.render(createElement(App));
+
+    unmountFunctions.push(() => root.unmount());
+  }
+}
+
+function createLegacyWrapper(App) {
+  return function LegacyApp() {
+    return createElement(App);
+  };
 }
 
 function mountTestApp() {
+  mountHelper(PartiallyStrictApp);
+  mountHelper(createLegacyWrapper(PartiallyStrictApp), true);
   mountHelper(ToDoList);
   mountHelper(InspectableElements);
   mountHelper(Hydration);
@@ -63,7 +80,7 @@ function mountTestApp() {
 }
 
 function unmountTestApp() {
-  roots.forEach(root => root.unmount());
+  unmountFunctions.forEach(fn => fn());
 }
 
 mountTestApp();
diff --git a/packages/react-devtools/OVERVIEW.md b/packages/react-devtools/OVERVIEW.md
index 70b2c9ba095b0..46612fcbf2e03 100644
--- a/packages/react-devtools/OVERVIEW.md
+++ b/packages/react-devtools/OVERVIEW.md
@@ -63,7 +63,9 @@ For example, adding a root fiber with an id of 1:
   1, // add operation
   1, // fiber id
   11, // ElementTypeRoot
+  1, // this root is StrictMode enabled
   1, // this root's renderer supports profiling
+  1, // this root's renderer supports StrictMode
   1, // this root has owner metadata
 ]
 ```