diff --git a/packages/react-devtools-shell/src/app/index.js b/packages/react-devtools-shell/src/app/index.js
index 69769f80d25a6..f991d61363bd1 100644
--- a/packages/react-devtools-shell/src/app/index.js
+++ b/packages/react-devtools-shell/src/app/index.js
@@ -69,6 +69,7 @@ function mountStrictApp(App) {
 }
 
 function mountLegacyApp(App: () => React$Node) {
+  // $FlowFixMe[prop-missing]: These are removed in 19.
   const {render, unmountComponentAtNode} = require('react-dom');
 
   function LegacyRender() {
@@ -77,8 +78,10 @@ function mountLegacyApp(App: () => React$Node) {
 
   const container = createContainer();
 
+  // $FlowFixMe[not-a-function]: These are removed in 19.
   render(createElement(LegacyRender), container);
 
+  // $FlowFixMe: These are removed in 19.
   unmountFunctions.push(() => unmountComponentAtNode(container));
 }
 
diff --git a/packages/react-devtools-shell/src/e2e-regression/app-legacy.js b/packages/react-devtools-shell/src/e2e-regression/app-legacy.js
index e8bb10fda091b..19ad2f0f6d953 100644
--- a/packages/react-devtools-shell/src/e2e-regression/app-legacy.js
+++ b/packages/react-devtools-shell/src/e2e-regression/app-legacy.js
@@ -15,6 +15,7 @@ function mountApp(App: () => React$Node) {
 
   ((document.body: any): HTMLBodyElement).appendChild(container);
 
+  // $FlowFixMe[prop-missing]: These are removed in 19.
   ReactDOM.render(<App />, container);
 }
 function mountTestApp() {
diff --git a/packages/react-dom/index.classic.fb.js b/packages/react-dom/index.classic.fb.js
index 5d4cb1e11f822..e765898d91cb8 100644
--- a/packages/react-dom/index.classic.fb.js
+++ b/packages/react-dom/index.classic.fb.js
@@ -20,11 +20,8 @@ Object.assign((Internals: any), {
 
 export {
   createPortal,
-  findDOMNode,
   flushSync,
-  unmountComponentAtNode,
   unstable_createEventHandle,
-  unstable_renderSubtreeIntoContainer,
   unstable_runWithPriority, // DO NOT USE: Temporarily exposed to migrate off of Scheduler.runWithPriority.
   useFormStatus,
   useFormState,
@@ -42,6 +39,9 @@ export {
   hydrateRoot,
   render,
   unstable_batchedUpdates,
+  findDOMNode,
+  unstable_renderSubtreeIntoContainer,
+  unmountComponentAtNode,
 } from './src/client/ReactDOMRootFB';
 
 export {Internals as __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED};
diff --git a/packages/react-dom/index.js b/packages/react-dom/index.js
index 30ccf0a2eac89..4357044d90818 100644
--- a/packages/react-dom/index.js
+++ b/packages/react-dom/index.js
@@ -15,11 +15,8 @@ export {
   createRoot,
   hydrateRoot,
   flushSync,
-  render,
-  unmountComponentAtNode,
   unstable_batchedUpdates,
   unstable_createEventHandle,
-  unstable_renderSubtreeIntoContainer,
   unstable_runWithPriority, // DO NOT USE: Temporarily exposed to migrate off of Scheduler.runWithPriority.
   useFormStatus,
   useFormState,
diff --git a/packages/react-dom/index.stable.js b/packages/react-dom/index.stable.js
index a817a6b3d9af3..6da13efa643cd 100644
--- a/packages/react-dom/index.stable.js
+++ b/packages/react-dom/index.stable.js
@@ -13,10 +13,7 @@ export {
   createRoot,
   hydrateRoot,
   flushSync,
-  render,
-  unmountComponentAtNode,
   unstable_batchedUpdates,
-  unstable_renderSubtreeIntoContainer,
   useFormStatus,
   useFormState,
   prefetchDNS,
diff --git a/packages/react-dom/src/ReactDOMSharedInternals.js b/packages/react-dom/src/ReactDOMSharedInternals.js
index 6de3648103591..7bd080ce48240 100644
--- a/packages/react-dom/src/ReactDOMSharedInternals.js
+++ b/packages/react-dom/src/ReactDOMSharedInternals.js
@@ -7,7 +7,6 @@
  * @flow
  */
 
-import type {FindDOMNodeType} from './client/ReactDOMLegacy.js';
 import type {HostDispatcher} from './shared/ReactDOMTypes';
 
 type InternalsType = {
@@ -16,7 +15,11 @@ type InternalsType = {
   ReactDOMCurrentDispatcher: {
     current: HostDispatcher,
   },
-  findDOMNode: null | FindDOMNodeType,
+  findDOMNode:
+    | null
+    | ((
+        componentOrElement: React$Component<any, any>,
+      ) => null | Element | Text),
 };
 
 function noop() {}
diff --git a/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js b/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
index f89e95bcf5dde..29e27cf35799e 100644
--- a/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
+++ b/packages/react-dom/src/__tests__/ReactComponentLifeCycle-test.js
@@ -14,7 +14,6 @@ let act;
 let React;
 let ReactDOM;
 let ReactDOMClient;
-let findDOMNode;
 
 const clone = function (o) {
   return JSON.parse(JSON.stringify(o));
@@ -95,8 +94,6 @@ describe('ReactComponentLifeCycle', () => {
 
     React = require('react');
     ReactDOM = require('react-dom');
-    findDOMNode =
-      ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.findDOMNode;
     ReactDOMClient = require('react-dom/client');
   });
 
@@ -376,6 +373,7 @@ describe('ReactComponentLifeCycle', () => {
     expect(instance.updater.isMounted(instance)).toBe(false);
   });
 
+  // @gate www && !disableLegacyMode
   it('warns if legacy findDOMNode is used inside render', async () => {
     class Component extends React.Component {
       state = {isMounted: false};
@@ -384,7 +382,7 @@ describe('ReactComponentLifeCycle', () => {
       }
       render() {
         if (this.state.isMounted) {
-          expect(findDOMNode(this).tagName).toBe('DIV');
+          expect(React.findDOMNode(this).tagName).toBe('DIV');
         }
         return <div />;
       }
diff --git a/packages/react-dom/src/__tests__/findDOMNode-test.js b/packages/react-dom/src/__tests__/findDOMNodeFB-test.js
similarity index 84%
rename from packages/react-dom/src/__tests__/findDOMNode-test.js
rename to packages/react-dom/src/__tests__/findDOMNodeFB-test.js
index 6dfcae82d4cfa..d89dd6cc3c9e2 100644
--- a/packages/react-dom/src/__tests__/findDOMNode-test.js
+++ b/packages/react-dom/src/__tests__/findDOMNodeFB-test.js
@@ -11,16 +11,15 @@
 
 const React = require('react');
 const ReactDOM = require('react-dom');
-const findDOMNode =
-  ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.findDOMNode;
 const StrictMode = React.StrictMode;
 
 describe('findDOMNode', () => {
+  // @gate www
   it('findDOMNode should return null if passed null', () => {
-    expect(findDOMNode(null)).toBe(null);
+    expect(ReactDOM.findDOMNode(null)).toBe(null);
   });
 
-  // @gate !disableLegacyMode
+  // @gate www && !disableLegacyMode
   it('findDOMNode should find dom element', () => {
     class MyNode extends React.Component {
       render() {
@@ -34,13 +33,13 @@ describe('findDOMNode', () => {
 
     const container = document.createElement('div');
     const myNode = ReactDOM.render(<MyNode />, container);
-    const myDiv = findDOMNode(myNode);
-    const mySameDiv = findDOMNode(myDiv);
+    const myDiv = ReactDOM.findDOMNode(myNode);
+    const mySameDiv = ReactDOM.findDOMNode(myDiv);
     expect(myDiv.tagName).toBe('DIV');
     expect(mySameDiv).toBe(myDiv);
   });
 
-  // @gate !disableLegacyMode
+  // @gate www && !disableLegacyMode
   it('findDOMNode should find dom element after an update from null', () => {
     function Bar({flag}) {
       if (flag) {
@@ -57,23 +56,24 @@ describe('findDOMNode', () => {
     const container = document.createElement('div');
 
     const myNodeA = ReactDOM.render(<MyNode />, container);
-    const a = findDOMNode(myNodeA);
+    const a = ReactDOM.findDOMNode(myNodeA);
     expect(a).toBe(null);
 
     const myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
     expect(myNodeA === myNodeB).toBe(true);
 
-    const b = findDOMNode(myNodeB);
+    const b = ReactDOM.findDOMNode(myNodeB);
     expect(b.tagName).toBe('SPAN');
   });
 
+  // @gate www
   it('findDOMNode should reject random objects', () => {
     expect(function () {
-      findDOMNode({foo: 'bar'});
+      ReactDOM.findDOMNode({foo: 'bar'});
     }).toThrowError('Argument appears to not be a ReactComponent. Keys: foo');
   });
 
-  // @gate !disableLegacyMode
+  // @gate www && !disableLegacyMode
   it('findDOMNode should reject unmounted objects with render func', () => {
     class Foo extends React.Component {
       render() {
@@ -85,16 +85,16 @@ describe('findDOMNode', () => {
     const inst = ReactDOM.render(<Foo />, container);
     ReactDOM.unmountComponentAtNode(container);
 
-    expect(() => findDOMNode(inst)).toThrowError(
+    expect(() => ReactDOM.findDOMNode(inst)).toThrowError(
       'Unable to find node on an unmounted component.',
     );
   });
 
-  // @gate !disableLegacyMode
+  // @gate www && !disableLegacyMode
   it('findDOMNode should not throw an error when called within a component that is not mounted', () => {
     class Bar extends React.Component {
       UNSAFE_componentWillMount() {
-        expect(findDOMNode(this)).toBeNull();
+        expect(ReactDOM.findDOMNode(this)).toBeNull();
       }
 
       render() {
@@ -107,7 +107,7 @@ describe('findDOMNode', () => {
     }).not.toThrow();
   });
 
-  // @gate !disableLegacyMode
+  // @gate www && !disableLegacyMode
   it('findDOMNode should warn if used to find a host component inside StrictMode', () => {
     let parent = undefined;
     let child = undefined;
@@ -129,7 +129,7 @@ describe('findDOMNode', () => {
     );
 
     let match;
-    expect(() => (match = findDOMNode(parent))).toErrorDev([
+    expect(() => (match = ReactDOM.findDOMNode(parent))).toErrorDev([
       'Warning: findDOMNode is deprecated in StrictMode. ' +
         'findDOMNode was passed an instance of ContainsStrictModeChild which renders StrictMode children. ' +
         'Instead, add a ref directly to the element you want to reference. ' +
@@ -141,7 +141,7 @@ describe('findDOMNode', () => {
     expect(match).toBe(child);
   });
 
-  // @gate !disableLegacyMode
+  // @gate www && !disableLegacyMode
   it('findDOMNode should warn if passed a component that is inside StrictMode', () => {
     let parent = undefined;
     let child = undefined;
@@ -162,7 +162,7 @@ describe('findDOMNode', () => {
     );
 
     let match;
-    expect(() => (match = findDOMNode(parent))).toErrorDev([
+    expect(() => (match = ReactDOM.findDOMNode(parent))).toErrorDev([
       'Warning: findDOMNode is deprecated in StrictMode. ' +
         'findDOMNode was passed an instance of IsInStrictMode which is inside StrictMode. ' +
         'Instead, add a ref directly to the element you want to reference. ' +
diff --git a/packages/react-dom/src/client/ReactDOM.js b/packages/react-dom/src/client/ReactDOM.js
index 99d2b62f2f845..e0cbe30a079f3 100644
--- a/packages/react-dom/src/client/ReactDOM.js
+++ b/packages/react-dom/src/client/ReactDOM.js
@@ -8,22 +8,12 @@
  */
 
 import type {ReactNodeList} from 'shared/ReactTypes';
-import type {
-  Container,
-  PublicInstance,
-} from 'react-dom-bindings/src/client/ReactFiberConfigDOM';
 import type {
   RootType,
   HydrateRootOptions,
   CreateRootOptions,
 } from './ReactDOMRoot';
 
-import {
-  findDOMNode,
-  render,
-  unstable_renderSubtreeIntoContainer,
-  unmountComponentAtNode,
-} from './ReactDOMLegacy';
 import {
   createRoot as createRootImpl,
   hydrateRoot as hydrateRootImpl,
@@ -35,6 +25,7 @@ import {
   flushSync as flushSyncWithoutWarningIfAlreadyRendering,
   isAlreadyRendering,
   injectIntoDevTools,
+  findHostInstance,
 } from 'react-reconciler/src/ReactFiberReconciler';
 import {runWithPriority} from 'react-reconciler/src/ReactEventPriorities';
 import {createPortal as createPortalImpl} from 'react-reconciler/src/ReactPortal';
@@ -99,20 +90,6 @@ function createPortal(
   return createPortalImpl(children, container, null, key);
 }
 
-function renderSubtreeIntoContainer(
-  parentComponent: React$Component<any, any>,
-  element: React$Element<any>,
-  containerNode: Container,
-  callback: ?Function,
-): React$Component<any, any> | PublicInstance | null {
-  return unstable_renderSubtreeIntoContainer(
-    parentComponent,
-    element,
-    containerNode,
-    callback,
-  );
-}
-
 function createRoot(
   container: Element | Document | DocumentFragment,
   options?: CreateRootOptions,
@@ -163,6 +140,12 @@ function flushSync<R>(fn: (() => R) | void): R | void {
   return flushSyncWithoutWarningIfAlreadyRendering(fn);
 }
 
+function findDOMNode(
+  componentOrElement: React$Component<any, any>,
+): null | Element | Text {
+  return findHostInstance(componentOrElement);
+}
+
 // Expose findDOMNode on internals
 Internals.findDOMNode = findDOMNode;
 
@@ -178,15 +161,9 @@ export {
   unstable_batchedUpdates,
   flushSync,
   ReactVersion as version,
-  // Disabled behind disableLegacyReactDOMAPIs
-  findDOMNode,
-  render,
-  unmountComponentAtNode,
   // exposeConcurrentModeAPIs
   createRoot,
   hydrateRoot,
-  // Disabled behind disableUnstableRenderSubtreeIntoContainer
-  renderSubtreeIntoContainer as unstable_renderSubtreeIntoContainer,
   // enableCreateEventHandleAPI
   createEventHandle as unstable_createEventHandle,
   // TODO: Remove this once callers migrate to alternatives.
diff --git a/packages/react-dom/src/client/ReactDOMLegacy.js b/packages/react-dom/src/client/ReactDOMLegacy.js
deleted file mode 100644
index 46220501a5e1e..0000000000000
--- a/packages/react-dom/src/client/ReactDOMLegacy.js
+++ /dev/null
@@ -1,434 +0,0 @@
-/**
- * Copyright (c) Meta Platforms, Inc. and 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 type {
-  Container,
-  PublicInstance,
-} from 'react-dom-bindings/src/client/ReactFiberConfigDOM';
-import type {FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
-import type {ReactNodeList} from 'shared/ReactTypes';
-
-import {disableLegacyMode} from 'shared/ReactFeatureFlags';
-import {clearContainer} from 'react-dom-bindings/src/client/ReactFiberConfigDOM';
-import {
-  getInstanceFromNode,
-  isContainerMarkedAsRoot,
-  markContainerAsRoot,
-  unmarkContainerAsRoot,
-} from 'react-dom-bindings/src/client/ReactDOMComponentTree';
-import {listenToAllSupportedEvents} from 'react-dom-bindings/src/events/DOMPluginEventSystem';
-import {isValidContainerLegacy} from './ReactDOMRoot';
-import {
-  DOCUMENT_NODE,
-  ELEMENT_NODE,
-  COMMENT_NODE,
-} from 'react-dom-bindings/src/client/HTMLNodeType';
-
-import {
-  createContainer,
-  createHydrationContainer,
-  findHostInstanceWithNoPortals,
-  updateContainer,
-  flushSync,
-  getPublicRootInstance,
-  findHostInstance,
-  findHostInstanceWithWarning,
-  defaultOnUncaughtError,
-  defaultOnCaughtError,
-} from 'react-reconciler/src/ReactFiberReconciler';
-import {LegacyRoot} from 'react-reconciler/src/ReactRootTags';
-import getComponentNameFromType from 'shared/getComponentNameFromType';
-import ReactSharedInternals from 'shared/ReactSharedInternals';
-import {has as hasInstance} from 'shared/ReactInstanceMap';
-
-const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;
-
-let topLevelUpdateWarnings;
-
-if (__DEV__) {
-  topLevelUpdateWarnings = (container: Container) => {
-    if (container._reactRootContainer && container.nodeType !== COMMENT_NODE) {
-      const hostInstance = findHostInstanceWithNoPortals(
-        container._reactRootContainer.current,
-      );
-      if (hostInstance) {
-        if (hostInstance.parentNode !== container) {
-          console.error(
-            'It looks like the React-rendered content of this ' +
-              'container was removed without using React. This is not ' +
-              'supported and will cause errors. Instead, call ' +
-              'ReactDOM.unmountComponentAtNode to empty a container.',
-          );
-        }
-      }
-    }
-
-    const isRootRenderedBySomeReact = !!container._reactRootContainer;
-    const rootEl = getReactRootElementInContainer(container);
-    const hasNonRootReactChild = !!(rootEl && getInstanceFromNode(rootEl));
-
-    if (hasNonRootReactChild && !isRootRenderedBySomeReact) {
-      console.error(
-        'Replacing React-rendered children with a new root ' +
-          'component. If you intended to update the children of this node, ' +
-          'you should instead have the existing children update their state ' +
-          'and render the new components instead of calling ReactDOM.render.',
-      );
-    }
-  };
-}
-
-function getReactRootElementInContainer(container: any) {
-  if (!container) {
-    return null;
-  }
-
-  if (container.nodeType === DOCUMENT_NODE) {
-    return container.documentElement;
-  } else {
-    return container.firstChild;
-  }
-}
-
-function noopOnRecoverableError() {
-  // This isn't reachable because onRecoverableError isn't called in the
-  // legacy API.
-}
-
-function legacyCreateRootFromDOMContainer(
-  container: Container,
-  initialChildren: ReactNodeList,
-  parentComponent: ?React$Component<any, any>,
-  callback: ?Function,
-  isHydrationContainer: boolean,
-): FiberRoot {
-  if (isHydrationContainer) {
-    if (typeof callback === 'function') {
-      const originalCallback = callback;
-      callback = function () {
-        const instance = getPublicRootInstance(root);
-        originalCallback.call(instance);
-      };
-    }
-
-    const root: FiberRoot = createHydrationContainer(
-      initialChildren,
-      callback,
-      container,
-      LegacyRoot,
-      null, // hydrationCallbacks
-      false, // isStrictMode
-      false, // concurrentUpdatesByDefaultOverride,
-      '', // identifierPrefix
-      defaultOnUncaughtError,
-      defaultOnCaughtError,
-      noopOnRecoverableError,
-      // TODO(luna) Support hydration later
-      null,
-      null,
-    );
-    container._reactRootContainer = root;
-    markContainerAsRoot(root.current, container);
-
-    const rootContainerElement =
-      container.nodeType === COMMENT_NODE ? container.parentNode : container;
-    // $FlowFixMe[incompatible-call]
-    listenToAllSupportedEvents(rootContainerElement);
-
-    flushSync();
-    return root;
-  } else {
-    // First clear any existing content.
-    clearContainer(container);
-
-    if (typeof callback === 'function') {
-      const originalCallback = callback;
-      callback = function () {
-        const instance = getPublicRootInstance(root);
-        originalCallback.call(instance);
-      };
-    }
-
-    const root = createContainer(
-      container,
-      LegacyRoot,
-      null, // hydrationCallbacks
-      false, // isStrictMode
-      false, // concurrentUpdatesByDefaultOverride,
-      '', // identifierPrefix
-      defaultOnUncaughtError,
-      defaultOnCaughtError,
-      noopOnRecoverableError,
-      null, // transitionCallbacks
-    );
-    container._reactRootContainer = root;
-    markContainerAsRoot(root.current, container);
-
-    const rootContainerElement =
-      container.nodeType === COMMENT_NODE ? container.parentNode : container;
-    // $FlowFixMe[incompatible-call]
-    listenToAllSupportedEvents(rootContainerElement);
-
-    // Initial mount should not be batched.
-    flushSync(() => {
-      updateContainer(initialChildren, root, parentComponent, callback);
-    });
-
-    return root;
-  }
-}
-
-function warnOnInvalidCallback(callback: mixed): void {
-  if (__DEV__) {
-    if (callback !== null && typeof callback !== 'function') {
-      console.error(
-        'Expected the last optional `callback` argument to be a ' +
-          'function. Instead received: %s.',
-        callback,
-      );
-    }
-  }
-}
-
-function legacyRenderSubtreeIntoContainer(
-  parentComponent: ?React$Component<any, any>,
-  children: ReactNodeList,
-  container: Container,
-  forceHydrate: boolean,
-  callback: ?Function,
-): React$Component<any, any> | PublicInstance | null {
-  if (__DEV__) {
-    topLevelUpdateWarnings(container);
-    warnOnInvalidCallback(callback === undefined ? null : callback);
-  }
-
-  const maybeRoot = container._reactRootContainer;
-  let root: FiberRoot;
-  if (!maybeRoot) {
-    // Initial mount
-    root = legacyCreateRootFromDOMContainer(
-      container,
-      children,
-      parentComponent,
-      callback,
-      forceHydrate,
-    );
-  } else {
-    root = maybeRoot;
-    if (typeof callback === 'function') {
-      const originalCallback = callback;
-      callback = function () {
-        const instance = getPublicRootInstance(root);
-        originalCallback.call(instance);
-      };
-    }
-    // Update
-    updateContainer(children, root, parentComponent, callback);
-  }
-  return getPublicRootInstance(root);
-}
-
-export type FindDOMNodeType = typeof findDOMNode;
-
-export function findDOMNode(
-  componentOrElement: Element | ?React$Component<any, any>,
-): null | Element | Text {
-  if (__DEV__) {
-    const owner = (ReactCurrentOwner.current: any);
-    if (owner !== null && owner.stateNode !== null) {
-      const warnedAboutRefsInRender = owner.stateNode._warnedAboutRefsInRender;
-      if (!warnedAboutRefsInRender) {
-        console.error(
-          '%s is accessing findDOMNode inside its render(). ' +
-            'render() should be a pure function of props and state. It should ' +
-            'never access something that requires stale data from the previous ' +
-            'render, such as refs. Move this logic to componentDidMount and ' +
-            'componentDidUpdate instead.',
-          getComponentNameFromType(owner.type) || 'A component',
-        );
-      }
-      owner.stateNode._warnedAboutRefsInRender = true;
-    }
-  }
-  if (componentOrElement == null) {
-    return null;
-  }
-  if ((componentOrElement: any).nodeType === ELEMENT_NODE) {
-    return (componentOrElement: any);
-  }
-  if (__DEV__) {
-    return findHostInstanceWithWarning(componentOrElement, 'findDOMNode');
-  }
-  return findHostInstance(componentOrElement);
-}
-
-export function render(
-  element: React$Element<any>,
-  container: Container,
-  callback: ?Function,
-): React$Component<any, any> | PublicInstance | null {
-  if (disableLegacyMode) {
-    if (__DEV__) {
-      console.error(
-        'ReactDOM.render was removed in React 19. Use createRoot instead.',
-      );
-    }
-    throw new Error('ReactDOM: Unsupported Legacy Mode API.');
-  }
-  if (__DEV__) {
-    console.error(
-      'ReactDOM.render has not been supported since React 18. Use createRoot ' +
-        'instead. Until you switch to the new API, your app will behave as ' +
-        "if it's running React 17. Learn " +
-        'more: https://react.dev/link/switch-to-createroot',
-    );
-  }
-
-  if (!isValidContainerLegacy(container)) {
-    throw new Error('Target container is not a DOM element.');
-  }
-
-  if (__DEV__) {
-    const isModernRoot =
-      isContainerMarkedAsRoot(container) &&
-      container._reactRootContainer === undefined;
-    if (isModernRoot) {
-      console.error(
-        'You are calling ReactDOM.render() on a container that was previously ' +
-          'passed to ReactDOMClient.createRoot(). This is not supported. ' +
-          'Did you mean to call root.render(element)?',
-      );
-    }
-  }
-  return legacyRenderSubtreeIntoContainer(
-    null,
-    element,
-    container,
-    false,
-    callback,
-  );
-}
-
-export function unstable_renderSubtreeIntoContainer(
-  parentComponent: React$Component<any, any>,
-  element: React$Element<any>,
-  containerNode: Container,
-  callback: ?Function,
-): React$Component<any, any> | PublicInstance | null {
-  if (disableLegacyMode) {
-    if (__DEV__) {
-      console.error(
-        'ReactDOM.unstable_renderSubtreeIntoContainer() was removed in React 19. Consider using a portal instead.',
-      );
-    }
-    throw new Error('ReactDOM: Unsupported Legacy Mode API.');
-  }
-  if (__DEV__) {
-    console.error(
-      'ReactDOM.unstable_renderSubtreeIntoContainer() has not been supported ' +
-        'since React 18. Consider using a portal instead. Until you switch to ' +
-        "the createRoot API, your app will behave as if it's running React " +
-        '17. Learn more: https://react.dev/link/switch-to-createroot',
-    );
-  }
-
-  if (!isValidContainerLegacy(containerNode)) {
-    throw new Error('Target container is not a DOM element.');
-  }
-
-  if (parentComponent == null || !hasInstance(parentComponent)) {
-    throw new Error('parentComponent must be a valid React Component');
-  }
-
-  return legacyRenderSubtreeIntoContainer(
-    parentComponent,
-    element,
-    containerNode,
-    false,
-    callback,
-  );
-}
-
-export function unmountComponentAtNode(container: Container): boolean {
-  if (disableLegacyMode) {
-    if (__DEV__) {
-      console.error(
-        'unmountComponentAtNode was removed in React 19. Use root.unmount() instead.',
-      );
-    }
-    throw new Error('ReactDOM: Unsupported Legacy Mode API.');
-  }
-  if (!isValidContainerLegacy(container)) {
-    throw new Error('Target container is not a DOM element.');
-  }
-
-  if (__DEV__) {
-    const isModernRoot =
-      isContainerMarkedAsRoot(container) &&
-      container._reactRootContainer === undefined;
-    if (isModernRoot) {
-      console.error(
-        'You are calling ReactDOM.unmountComponentAtNode() on a container that was previously ' +
-          'passed to ReactDOMClient.createRoot(). This is not supported. Did you mean to call root.unmount()?',
-      );
-    }
-  }
-
-  if (container._reactRootContainer) {
-    if (__DEV__) {
-      const rootEl = getReactRootElementInContainer(container);
-      const renderedByDifferentReact = rootEl && !getInstanceFromNode(rootEl);
-      if (renderedByDifferentReact) {
-        console.error(
-          "unmountComponentAtNode(): The node you're attempting to unmount " +
-            'was rendered by another copy of React.',
-        );
-      }
-    }
-
-    // Unmount should not be batched.
-    flushSync(() => {
-      legacyRenderSubtreeIntoContainer(null, null, container, false, () => {
-        // $FlowFixMe[incompatible-type] This should probably use `delete container._reactRootContainer`
-        container._reactRootContainer = null;
-        unmarkContainerAsRoot(container);
-      });
-    });
-    // If you call unmountComponentAtNode twice in quick succession, you'll
-    // get `true` twice. That's probably fine?
-    return true;
-  } else {
-    if (__DEV__) {
-      const rootEl = getReactRootElementInContainer(container);
-      const hasNonRootReactChild = !!(rootEl && getInstanceFromNode(rootEl));
-
-      // Check if the container itself is a React root node.
-      const isContainerReactRoot =
-        container.nodeType === ELEMENT_NODE &&
-        isValidContainerLegacy(container.parentNode) &&
-        // $FlowFixMe[prop-missing]
-        // $FlowFixMe[incompatible-use]
-        !!container.parentNode._reactRootContainer;
-
-      if (hasNonRootReactChild) {
-        console.error(
-          "unmountComponentAtNode(): The node you're attempting to unmount " +
-            'was rendered by React and is not a top-level container. %s',
-          isContainerReactRoot
-            ? 'You may have accidentally passed in a React root node instead ' +
-                'of its container.'
-            : 'Instead, have the parent component update its state and ' +
-                'rerender in order to remove this component.',
-        );
-      }
-    }
-
-    return false;
-  }
-}
diff --git a/packages/react-dom/src/client/ReactDOMRootFB.js b/packages/react-dom/src/client/ReactDOMRootFB.js
index 138f781e6caa7..4bf5b43f6e51d 100644
--- a/packages/react-dom/src/client/ReactDOMRootFB.js
+++ b/packages/react-dom/src/client/ReactDOMRootFB.js
@@ -33,11 +33,13 @@ import {
   getInstanceFromNode,
   isContainerMarkedAsRoot,
   markContainerAsRoot,
+  unmarkContainerAsRoot,
 } from 'react-dom-bindings/src/client/ReactDOMComponentTree';
 import {listenToAllSupportedEvents} from 'react-dom-bindings/src/events/DOMPluginEventSystem';
 import {isValidContainerLegacy} from './ReactDOMRoot';
 import {
   DOCUMENT_NODE,
+  ELEMENT_NODE,
   COMMENT_NODE,
 } from 'react-dom-bindings/src/client/HTMLNodeType';
 
@@ -49,12 +51,17 @@ import {
   updateContainer,
   flushSync,
   getPublicRootInstance,
+  findHostInstance,
+  findHostInstanceWithWarning,
   defaultOnUncaughtError,
   defaultOnCaughtError,
 } from 'react-reconciler/src/ReactFiberReconciler';
 import {LegacyRoot} from 'react-reconciler/src/ReactRootTags';
+import getComponentNameFromType from 'shared/getComponentNameFromType';
 import {has as hasInstance} from 'shared/ReactInstanceMap';
 
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
 import assign from 'shared/assign';
 
 // Provided by www
@@ -146,6 +153,8 @@ export function hydrateRoot(
   );
 }
 
+const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;
+
 let topLevelUpdateWarnings;
 
 if (__DEV__) {
@@ -331,6 +340,38 @@ function legacyRenderSubtreeIntoContainer(
   return getPublicRootInstance(root);
 }
 
+export function findDOMNode(
+  componentOrElement: Element | ?React$Component<any, any>,
+): null | Element | Text {
+  if (__DEV__) {
+    const owner = (ReactCurrentOwner.current: any);
+    if (owner !== null && owner.stateNode !== null) {
+      const warnedAboutRefsInRender = owner.stateNode._warnedAboutRefsInRender;
+      if (!warnedAboutRefsInRender) {
+        console.error(
+          '%s is accessing findDOMNode inside its render(). ' +
+            'render() should be a pure function of props and state. It should ' +
+            'never access something that requires stale data from the previous ' +
+            'render, such as refs. Move this logic to componentDidMount and ' +
+            'componentDidUpdate instead.',
+          getComponentNameFromType(owner.type) || 'A component',
+        );
+      }
+      owner.stateNode._warnedAboutRefsInRender = true;
+    }
+  }
+  if (componentOrElement == null) {
+    return null;
+  }
+  if ((componentOrElement: any).nodeType === ELEMENT_NODE) {
+    return (componentOrElement: any);
+  }
+  if (__DEV__) {
+    return findHostInstanceWithWarning(componentOrElement, 'findDOMNode');
+  }
+  return findHostInstance(componentOrElement);
+}
+
 export function render(
   element: React$Element<any>,
   container: Container,
@@ -418,4 +459,82 @@ export function unstable_renderSubtreeIntoContainer(
   );
 }
 
+export function unmountComponentAtNode(container: Container): boolean {
+  if (disableLegacyMode) {
+    if (__DEV__) {
+      console.error(
+        'unmountComponentAtNode was removed in React 19. Use root.unmount() instead.',
+      );
+    }
+    throw new Error('ReactDOM: Unsupported Legacy Mode API.');
+  }
+  if (!isValidContainerLegacy(container)) {
+    throw new Error('Target container is not a DOM element.');
+  }
+
+  if (__DEV__) {
+    const isModernRoot =
+      isContainerMarkedAsRoot(container) &&
+      container._reactRootContainer === undefined;
+    if (isModernRoot) {
+      console.error(
+        'You are calling ReactDOM.unmountComponentAtNode() on a container that was previously ' +
+          'passed to ReactDOMClient.createRoot(). This is not supported. Did you mean to call root.unmount()?',
+      );
+    }
+  }
+
+  if (container._reactRootContainer) {
+    if (__DEV__) {
+      const rootEl = getReactRootElementInContainer(container);
+      const renderedByDifferentReact = rootEl && !getInstanceFromNode(rootEl);
+      if (renderedByDifferentReact) {
+        console.error(
+          "unmountComponentAtNode(): The node you're attempting to unmount " +
+            'was rendered by another copy of React.',
+        );
+      }
+    }
+
+    // Unmount should not be batched.
+    flushSync(() => {
+      legacyRenderSubtreeIntoContainer(null, null, container, false, () => {
+        // $FlowFixMe[incompatible-type] This should probably use `delete container._reactRootContainer`
+        container._reactRootContainer = null;
+        unmarkContainerAsRoot(container);
+      });
+    });
+    // If you call unmountComponentAtNode twice in quick succession, you'll
+    // get `true` twice. That's probably fine?
+    return true;
+  } else {
+    if (__DEV__) {
+      const rootEl = getReactRootElementInContainer(container);
+      const hasNonRootReactChild = !!(rootEl && getInstanceFromNode(rootEl));
+
+      // Check if the container itself is a React root node.
+      const isContainerReactRoot =
+        container.nodeType === ELEMENT_NODE &&
+        isValidContainerLegacy(container.parentNode) &&
+        // $FlowFixMe[prop-missing]
+        // $FlowFixMe[incompatible-use]
+        !!container.parentNode._reactRootContainer;
+
+      if (hasNonRootReactChild) {
+        console.error(
+          "unmountComponentAtNode(): The node you're attempting to unmount " +
+            'was rendered by React and is not a top-level container. %s',
+          isContainerReactRoot
+            ? 'You may have accidentally passed in a React root node instead ' +
+                'of its container.'
+            : 'Instead, have the parent component update its state and ' +
+                'rerender in order to remove this component.',
+        );
+      }
+    }
+
+    return false;
+  }
+}
+
 export {batchedUpdates as unstable_batchedUpdates};
diff --git a/packages/react-dom/unstable_testing.js b/packages/react-dom/unstable_testing.js
index 1e973748c7cb8..19cc1515cdffe 100644
--- a/packages/react-dom/unstable_testing.js
+++ b/packages/react-dom/unstable_testing.js
@@ -10,11 +10,8 @@
 export {
   createPortal,
   flushSync,
-  render,
-  unmountComponentAtNode,
   unstable_batchedUpdates,
   unstable_createEventHandle,
-  unstable_renderSubtreeIntoContainer,
   unstable_runWithPriority, // DO NOT USE: Temporarily exposed to migrate off of Scheduler.runWithPriority.
   useFormStatus,
   useFormState,
diff --git a/packages/react-dom/unstable_testing.stable.js b/packages/react-dom/unstable_testing.stable.js
index 0b47d34c0acb0..a0a460ce83a99 100644
--- a/packages/react-dom/unstable_testing.stable.js
+++ b/packages/react-dom/unstable_testing.stable.js
@@ -10,10 +10,7 @@
 export {
   createPortal,
   flushSync,
-  render,
-  unmountComponentAtNode,
   unstable_batchedUpdates,
-  unstable_renderSubtreeIntoContainer,
   useFormStatus,
   useFormState,
   prefetchDNS,
diff --git a/packages/react/src/__tests__/ReactJSXRuntime-test.js b/packages/react/src/__tests__/ReactJSXRuntime-test.js
index db488e1684001..16acc60aef5b0 100644
--- a/packages/react/src/__tests__/ReactJSXRuntime-test.js
+++ b/packages/react/src/__tests__/ReactJSXRuntime-test.js
@@ -15,7 +15,6 @@ let ReactDOMClient;
 let JSXRuntime;
 let JSXDEVRuntime;
 let act;
-let findDOMNode;
 
 // NOTE: Prefer to call the JSXRuntime directly in these tests so we can be
 // certain that we are testing the runtime behavior, as opposed to the Babel
@@ -30,8 +29,6 @@ describe('ReactJSXRuntime', () => {
     ReactDOM = require('react-dom');
     ReactDOMClient = require('react-dom/client');
     act = require('internal-test-utils').act;
-    findDOMNode =
-      ReactDOM.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.findDOMNode;
   });
 
   it('allows static methods to be called using the type property', () => {
@@ -133,9 +130,9 @@ describe('ReactJSXRuntime', () => {
 
     const outer = container.firstChild;
     if (__DEV__) {
-      expect(findDOMNode(outer).className).toBe('moo');
+      expect(outer.className).toBe('moo');
     } else {
-      expect(findDOMNode(outer).className).toBe('quack');
+      expect(outer.className).toBe('quack');
     }
   });
 
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index 7113757b66676..e9f25eea1dc69 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -178,7 +178,7 @@ export const enableReactTestRendererWarning = __NEXT_MAJOR__;
 // Disables legacy mode
 // This allows us to land breaking changes to remove legacy mode APIs in experimental builds
 // before removing them in stable in the next Major
-export const disableLegacyMode = __NEXT_MAJOR__;
+export const disableLegacyMode = true;
 
 export const disableDOMTestUtils = __NEXT_MAJOR__;