diff --git a/packages/react-dom/index.classic.fb.js b/packages/react-dom/index.classic.fb.js
index 234aa33066485..5f4f04e634615 100644
--- a/packages/react-dom/index.classic.fb.js
+++ b/packages/react-dom/index.classic.fb.js
@@ -23,6 +23,7 @@ export {
   createPortal,
   createRoot,
   createRoot as unstable_createRoot, // TODO Remove once callsites use createRoot
+  hydrateRoot,
   findDOMNode,
   flushSync,
   hydrate,
diff --git a/packages/react-dom/index.experimental.js b/packages/react-dom/index.experimental.js
index 5a1c9f191f702..5cb691740e714 100644
--- a/packages/react-dom/index.experimental.js
+++ b/packages/react-dom/index.experimental.js
@@ -11,6 +11,7 @@ export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
   createPortal,
   createRoot,
+  hydrateRoot,
   findDOMNode,
   flushSync,
   hydrate,
diff --git a/packages/react-dom/index.js b/packages/react-dom/index.js
index 6a824b2f70b14..7b89695091d54 100644
--- a/packages/react-dom/index.js
+++ b/packages/react-dom/index.js
@@ -14,6 +14,7 @@ export {
   createPortal,
   createRoot,
   createRoot as unstable_createRoot,
+  hydrateRoot,
   findDOMNode,
   flushSync,
   hydrate,
diff --git a/packages/react-dom/index.modern.fb.js b/packages/react-dom/index.modern.fb.js
index b53a8cf9add79..b12c249286b1b 100644
--- a/packages/react-dom/index.modern.fb.js
+++ b/packages/react-dom/index.modern.fb.js
@@ -12,6 +12,7 @@ export {
   createPortal,
   createRoot,
   createRoot as unstable_createRoot, // TODO Remove once callsites use createRoot
+  hydrateRoot,
   flushSync,
   unstable_batchedUpdates,
   unstable_createEventHandle,
diff --git a/packages/react-dom/index.stable.js b/packages/react-dom/index.stable.js
index 55d515a9fce2f..7d8b01be53e3d 100644
--- a/packages/react-dom/index.stable.js
+++ b/packages/react-dom/index.stable.js
@@ -11,6 +11,7 @@ export {
   __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED,
   createPortal,
   createRoot,
+  hydrateRoot,
   findDOMNode,
   flushSync,
   hydrate,
diff --git a/packages/react-dom/src/client/ReactDOM.js b/packages/react-dom/src/client/ReactDOM.js
index 848e889a35dbe..7355e0be45a3c 100644
--- a/packages/react-dom/src/client/ReactDOM.js
+++ b/packages/react-dom/src/client/ReactDOM.js
@@ -17,7 +17,7 @@ import {
   unstable_renderSubtreeIntoContainer,
   unmountComponentAtNode,
 } from './ReactDOMLegacy';
-import {createRoot, isValidContainer} from './ReactDOMRoot';
+import {createRoot, hydrateRoot, isValidContainer} from './ReactDOMRoot';
 import {createEventHandle} from './ReactDOMEventHandle';
 
 import {
@@ -182,6 +182,7 @@ export {
   unmountComponentAtNode,
   // exposeConcurrentModeAPIs
   createRoot,
+  hydrateRoot,
   flushControlled as unstable_flushControlled,
   scheduleHydration as unstable_scheduleHydration,
   // Disabled behind disableUnstableRenderSubtreeIntoContainer
diff --git a/packages/react-dom/src/client/ReactDOMHostConfig.js b/packages/react-dom/src/client/ReactDOMHostConfig.js
index 908719d679891..cd8f913b9d513 100644
--- a/packages/react-dom/src/client/ReactDOMHostConfig.js
+++ b/packages/react-dom/src/client/ReactDOMHostConfig.js
@@ -105,8 +105,8 @@ export type EventTargetChildElement = {
   ...
 };
 export type Container =
-  | (Element & {_reactRootContainer?: RootType, ...})
-  | (Document & {_reactRootContainer?: RootType, ...});
+  | (Element & {_reactRootContainer?: FiberRoot, ...})
+  | (Document & {_reactRootContainer?: FiberRoot, ...});
 export type Instance = Element;
 export type TextInstance = Text;
 export type SuspenseInstance = Comment & {_reactRetry?: () => void, ...};
diff --git a/packages/react-dom/src/client/ReactDOMLegacy.js b/packages/react-dom/src/client/ReactDOMLegacy.js
index e85a5541b19e6..e6253b19dde6e 100644
--- a/packages/react-dom/src/client/ReactDOMLegacy.js
+++ b/packages/react-dom/src/client/ReactDOMLegacy.js
@@ -8,16 +8,17 @@
  */
 
 import type {Container} from './ReactDOMHostConfig';
-import type {RootType} from './ReactDOMRoot';
 import type {FiberRoot} from 'react-reconciler/src/ReactInternalTypes';
 import type {ReactNodeList} from 'shared/ReactTypes';
 
 import {
   getInstanceFromNode,
   isContainerMarkedAsRoot,
+  markContainerAsRoot,
   unmarkContainerAsRoot,
 } from './ReactDOMComponentTree';
-import {createLegacyRoot, isValidContainer} from './ReactDOMRoot';
+import {listenToAllSupportedEvents} from '../events/DOMPluginEventSystem';
+import {isValidContainerLegacy} from './ReactDOMRoot';
 import {
   DOCUMENT_NODE,
   ELEMENT_NODE,
@@ -25,6 +26,7 @@ import {
 } from '../shared/HTMLNodeType';
 
 import {
+  createContainer,
   findHostInstanceWithNoPortals,
   updateContainer,
   unbatchedUpdates,
@@ -32,6 +34,7 @@ import {
   findHostInstance,
   findHostInstanceWithWarning,
 } from 'react-reconciler/src/ReactFiberReconciler';
+import {LegacyRoot} from 'react-reconciler/src/ReactRootTags';
 import getComponentNameFromType from 'shared/getComponentNameFromType';
 import invariant from 'shared/invariant';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
@@ -45,7 +48,7 @@ if (__DEV__) {
   topLevelUpdateWarnings = (container: Container) => {
     if (container._reactRootContainer && container.nodeType !== COMMENT_NODE) {
       const hostInstance = findHostInstanceWithNoPortals(
-        container._reactRootContainer._internalRoot.current,
+        container._reactRootContainer.current,
       );
       if (hostInstance) {
         if (hostInstance.parentNode !== container) {
@@ -103,7 +106,7 @@ function getReactRootElementInContainer(container: any) {
 function legacyCreateRootFromDOMContainer(
   container: Container,
   forceHydrate: boolean,
-): RootType {
+): FiberRoot {
   // First clear any existing content.
   if (!forceHydrate) {
     let rootSibling;
@@ -112,14 +115,21 @@ function legacyCreateRootFromDOMContainer(
     }
   }
 
-  return createLegacyRoot(
+  const root = createContainer(
     container,
-    forceHydrate
-      ? {
-          hydrate: true,
-        }
-      : undefined,
+    LegacyRoot,
+    forceHydrate,
+    null, // hydrationCallbacks
+    false, // isStrictMode
+    false, // concurrentUpdatesByDefaultOverride,
   );
+  markContainerAsRoot(root.current, container);
+
+  const rootContainerElement =
+    container.nodeType === COMMENT_NODE ? container.parentNode : container;
+  listenToAllSupportedEvents(rootContainerElement);
+
+  return root;
 }
 
 function warnOnInvalidCallback(callback: mixed, callerName: string): void {
@@ -155,7 +165,7 @@ function legacyRenderSubtreeIntoContainer(
       container,
       forceHydrate,
     );
-    fiberRoot = root._internalRoot;
+    fiberRoot = root;
     if (typeof callback === 'function') {
       const originalCallback = callback;
       callback = function() {
@@ -168,7 +178,7 @@ function legacyRenderSubtreeIntoContainer(
       updateContainer(children, fiberRoot, parentComponent, callback);
     });
   } else {
-    fiberRoot = root._internalRoot;
+    fiberRoot = root;
     if (typeof callback === 'function') {
       const originalCallback = callback;
       callback = function() {
@@ -221,7 +231,7 @@ export function hydrate(
 ) {
   if (__DEV__) {
     console.error(
-      'ReactDOM.hydrate is no longer supported in React 18. Use createRoot ' +
+      'ReactDOM.hydrate is no longer supported in React 18. Use hydrateRoot ' +
         'instead. Until you switch to the new API, your app will behave as ' +
         "if it's running React 17. Learn " +
         'more: https://reactjs.org/link/switch-to-createroot',
@@ -229,7 +239,7 @@ export function hydrate(
   }
 
   invariant(
-    isValidContainer(container),
+    isValidContainerLegacy(container),
     'Target container is not a DOM element.',
   );
   if (__DEV__) {
@@ -240,7 +250,7 @@ export function hydrate(
       console.error(
         'You are calling ReactDOM.hydrate() on a container that was previously ' +
           'passed to ReactDOM.createRoot(). This is not supported. ' +
-          'Did you mean to call createRoot(container, {hydrate: true}).render(element)?',
+          'Did you mean to call hydrateRoot(container).render(element)?',
       );
     }
   }
@@ -269,7 +279,7 @@ export function render(
   }
 
   invariant(
-    isValidContainer(container),
+    isValidContainerLegacy(container),
     'Target container is not a DOM element.',
   );
   if (__DEV__) {
@@ -300,7 +310,7 @@ export function unstable_renderSubtreeIntoContainer(
   callback: ?Function,
 ) {
   invariant(
-    isValidContainer(containerNode),
+    isValidContainerLegacy(containerNode),
     'Target container is not a DOM element.',
   );
   invariant(
@@ -318,7 +328,7 @@ export function unstable_renderSubtreeIntoContainer(
 
 export function unmountComponentAtNode(container: Container) {
   invariant(
-    isValidContainer(container),
+    isValidContainerLegacy(container),
     'unmountComponentAtNode(...): Target container is not a DOM element.',
   );
 
@@ -365,7 +375,7 @@ export function unmountComponentAtNode(container: Container) {
       // Check if the container itself is a React root node.
       const isContainerReactRoot =
         container.nodeType === ELEMENT_NODE &&
-        isValidContainer(container.parentNode) &&
+        isValidContainerLegacy(container.parentNode) &&
         !!container.parentNode._reactRootContainer;
 
       if (hasNonRootReactChild) {
diff --git a/packages/react-dom/src/client/ReactDOMRoot.js b/packages/react-dom/src/client/ReactDOMRoot.js
index dc4c95a41ef30..616264f1502c0 100644
--- a/packages/react-dom/src/client/ReactDOMRoot.js
+++ b/packages/react-dom/src/client/ReactDOMRoot.js
@@ -19,7 +19,8 @@ export type RootType = {
   ...
 };
 
-export type RootOptions = {
+export type CreateRootOptions = {
+  // TODO: Remove these options.
   hydrate?: boolean,
   hydrationOptions?: {
     onHydrated?: (suspenseNode: Comment) => void,
@@ -27,6 +28,18 @@ export type RootOptions = {
     mutableSources?: Array<MutableSource<any>>,
     ...
   },
+  // END OF TODO
+  unstable_strictMode?: boolean,
+  unstable_concurrentUpdatesByDefault?: boolean,
+  ...
+};
+
+export type HydrateRootOptions = {
+  // Hydration options
+  hydratedSources?: Array<MutableSource<any>>,
+  onHydrated?: (suspenseNode: Comment) => void,
+  onDeleted?: (suspenseNode: Comment) => void,
+  // Options for all roots
   unstable_strictMode?: boolean,
   unstable_concurrentUpdatesByDefault?: boolean,
   ...
@@ -52,20 +65,14 @@ import {
   registerMutableSourceForHydration,
 } from 'react-reconciler/src/ReactFiberReconciler';
 import invariant from 'shared/invariant';
-import {ConcurrentRoot, LegacyRoot} from 'react-reconciler/src/ReactRootTags';
+import {ConcurrentRoot} from 'react-reconciler/src/ReactRootTags';
 import {allowConcurrentByDefault} from 'shared/ReactFeatureFlags';
 
-function ReactDOMRoot(container: Container, options: void | RootOptions) {
-  this._internalRoot = createRootImpl(container, ConcurrentRoot, options);
-}
-
-function ReactDOMLegacyRoot(container: Container, options: void | RootOptions) {
-  this._internalRoot = createRootImpl(container, LegacyRoot, options);
+function ReactDOMRoot(internalRoot) {
+  this._internalRoot = internalRoot;
 }
 
-ReactDOMRoot.prototype.render = ReactDOMLegacyRoot.prototype.render = function(
-  children: ReactNodeList,
-): void {
+ReactDOMRoot.prototype.render = function(children: ReactNodeList): void {
   const root = this._internalRoot;
   if (__DEV__) {
     if (typeof arguments[1] === 'function') {
@@ -93,7 +100,7 @@ ReactDOMRoot.prototype.render = ReactDOMLegacyRoot.prototype.render = function(
   updateContainer(children, root, null, null);
 };
 
-ReactDOMRoot.prototype.unmount = ReactDOMLegacyRoot.prototype.unmount = function(): void {
+ReactDOMRoot.prototype.unmount = function(): void {
   if (__DEV__) {
     if (typeof arguments[0] === 'function') {
       console.error(
@@ -109,12 +116,17 @@ ReactDOMRoot.prototype.unmount = ReactDOMLegacyRoot.prototype.unmount = function
   });
 };
 
-function createRootImpl(
+export function createRoot(
   container: Container,
-  tag: RootTag,
-  options: void | RootOptions,
-) {
-  // Tag is either LegacyRoot or Concurrent Root
+  options?: CreateRootOptions,
+): RootType {
+  invariant(
+    isValidContainerLegacy(container),
+    'createRoot(...): Target container is not a DOM element.',
+  );
+  warnIfReactDOMContainerInDEV(container);
+
+  // TODO: Delete these options
   const hydrate = options != null && options.hydrate === true;
   const hydrationCallbacks =
     (options != null && options.hydrationOptions) || null;
@@ -123,8 +135,9 @@ function createRootImpl(
       options.hydrationOptions != null &&
       options.hydrationOptions.mutableSources) ||
     null;
-  const isStrictMode = options != null && options.unstable_strictMode === true;
+  // END TODO
 
+  const isStrictMode = options != null && options.unstable_strictMode === true;
   let concurrentUpdatesByDefaultOverride = null;
   if (allowConcurrentByDefault) {
     concurrentUpdatesByDefaultOverride =
@@ -135,7 +148,7 @@ function createRootImpl(
 
   const root = createContainer(
     container,
-    tag,
+    ConcurrentRoot,
     hydrate,
     hydrationCallbacks,
     isStrictMode,
@@ -147,36 +160,80 @@ function createRootImpl(
     container.nodeType === COMMENT_NODE ? container.parentNode : container;
   listenToAllSupportedEvents(rootContainerElement);
 
+  // TODO: Delete this path
   if (mutableSources) {
     for (let i = 0; i < mutableSources.length; i++) {
       const mutableSource = mutableSources[i];
       registerMutableSourceForHydration(root, mutableSource);
     }
   }
+  // END TODO
 
-  return root;
+  return new ReactDOMRoot(root);
 }
 
-export function createRoot(
+export function hydrateRoot(
   container: Container,
-  options?: RootOptions,
+  initialChildren: ReactNodeList,
+  options?: HydrateRootOptions,
 ): RootType {
   invariant(
     isValidContainer(container),
-    'createRoot(...): Target container is not a DOM element.',
+    'hydrateRoot(...): Target container is not a DOM element.',
   );
   warnIfReactDOMContainerInDEV(container);
-  return new ReactDOMRoot(container, options);
+
+  // For now we reuse the whole bag of options since they contain
+  // the hydration callbacks.
+  const hydrationCallbacks = options != null ? options : null;
+  const mutableSources = (options != null && options.hydratedSources) || null;
+  const isStrictMode = options != null && options.unstable_strictMode === true;
+
+  let concurrentUpdatesByDefaultOverride = null;
+  if (allowConcurrentByDefault) {
+    concurrentUpdatesByDefaultOverride =
+      options != null && options.unstable_concurrentUpdatesByDefault != null
+        ? options.unstable_concurrentUpdatesByDefault
+        : null;
+  }
+
+  const root = createContainer(
+    container,
+    ConcurrentRoot,
+    true, // hydrate
+    hydrationCallbacks,
+    isStrictMode,
+    concurrentUpdatesByDefaultOverride,
+  );
+  markContainerAsRoot(root.current, container);
+  // This can't be a comment node since hydration doesn't work on comment nodes anyway.
+  listenToAllSupportedEvents(container);
+
+  if (mutableSources) {
+    for (let i = 0; i < mutableSources.length; i++) {
+      const mutableSource = mutableSources[i];
+      registerMutableSourceForHydration(root, mutableSource);
+    }
+  }
+
+  // Render the initial children
+  updateContainer(initialChildren, root, null, null);
+
+  return new ReactDOMRoot(root);
 }
 
-export function createLegacyRoot(
-  container: Container,
-  options?: RootOptions,
-): RootType {
-  return new ReactDOMLegacyRoot(container, options);
+export function isValidContainer(node: any): boolean {
+  return !!(
+    node &&
+    (node.nodeType === ELEMENT_NODE ||
+      node.nodeType === DOCUMENT_NODE ||
+      node.nodeType === DOCUMENT_FRAGMENT_NODE)
+  );
 }
 
-export function isValidContainer(node: mixed): boolean {
+// TODO: Remove this function which also includes comment nodes.
+// We only use it in places that are currently more relaxed.
+export function isValidContainerLegacy(node: any): boolean {
   return !!(
     node &&
     (node.nodeType === ELEMENT_NODE ||