diff --git a/packages/react-art/src/ReactART.js b/packages/react-art/src/ReactART.js
index 9d1b6a16c2038..44cdf4f240a21 100644
--- a/packages/react-art/src/ReactART.js
+++ b/packages/react-art/src/ReactART.js
@@ -69,7 +69,6 @@ class Surface extends React.Component {
     this._mountNode = createContainer(
       this._surface,
       LegacyRoot,
-      false,
       null,
       false,
       false,
diff --git a/packages/react-dom/src/__tests__/ReactDOMRoot-test.js b/packages/react-dom/src/__tests__/ReactDOMRoot-test.js
index df693b8784992..9d6a38188376d 100644
--- a/packages/react-dom/src/__tests__/ReactDOMRoot-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMRoot-test.js
@@ -253,6 +253,15 @@ describe('ReactDOMRoot', () => {
     );
   });
 
+  it('callback passed to legacy hydrate() API', () => {
+    container.innerHTML = '<div>Hi</div>';
+    ReactDOM.hydrate(<div>Hi</div>, container, () => {
+      Scheduler.unstable_yieldValue('callback');
+    });
+    expect(container.textContent).toEqual('Hi');
+    expect(Scheduler).toHaveYielded(['callback']);
+  });
+
   it('warns when unmounting with legacy API (no previous content)', () => {
     const root = ReactDOMClient.createRoot(container);
     root.render(<div>Hi</div>);
diff --git a/packages/react-dom/src/client/ReactDOMLegacy.js b/packages/react-dom/src/client/ReactDOMLegacy.js
index 3b751405a3034..af0e35e128bd4 100644
--- a/packages/react-dom/src/client/ReactDOMLegacy.js
+++ b/packages/react-dom/src/client/ReactDOMLegacy.js
@@ -27,6 +27,7 @@ import {
 
 import {
   createContainer,
+  createHydrationContainer,
   findHostInstanceWithNoPortals,
   updateContainer,
   flushSync,
@@ -109,34 +110,81 @@ function noopOnRecoverableError() {
 
 function legacyCreateRootFromDOMContainer(
   container: Container,
-  forceHydrate: boolean,
+  initialChildren: ReactNodeList,
+  parentComponent: ?React$Component<any, any>,
+  callback: ?Function,
+  isHydrationContainer: boolean,
 ): FiberRoot {
-  // First clear any existing content.
-  if (!forceHydrate) {
+  if (isHydrationContainer) {
+    if (typeof callback === 'function') {
+      const originalCallback = callback;
+      callback = function() {
+        const instance = getPublicRootInstance(root);
+        originalCallback.call(instance);
+      };
+    }
+
+    const root = createHydrationContainer(
+      initialChildren,
+      callback,
+      container,
+      LegacyRoot,
+      null, // hydrationCallbacks
+      false, // isStrictMode
+      false, // concurrentUpdatesByDefaultOverride,
+      '', // identifierPrefix
+      noopOnRecoverableError,
+      // TODO(luna) Support hydration later
+      null,
+    );
+    container._reactRootContainer = root;
+    markContainerAsRoot(root.current, container);
+
+    const rootContainerElement =
+      container.nodeType === COMMENT_NODE ? container.parentNode : container;
+    listenToAllSupportedEvents(rootContainerElement);
+
+    flushSync();
+    return root;
+  } else {
+    // First clear any existing content.
     let rootSibling;
     while ((rootSibling = container.lastChild)) {
       container.removeChild(rootSibling);
     }
-  }
 
-  const root = createContainer(
-    container,
-    LegacyRoot,
-    forceHydrate,
-    null, // hydrationCallbacks
-    false, // isStrictMode
-    false, // concurrentUpdatesByDefaultOverride,
-    '', // identifierPrefix
-    noopOnRecoverableError, // onRecoverableError
-    null, // transitionCallbacks
-  );
-  markContainerAsRoot(root.current, 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
+      noopOnRecoverableError, // onRecoverableError
+      null, // transitionCallbacks
+    );
+    container._reactRootContainer = root;
+    markContainerAsRoot(root.current, container);
+
+    const rootContainerElement =
+      container.nodeType === COMMENT_NODE ? container.parentNode : container;
+    listenToAllSupportedEvents(rootContainerElement);
 
-  const rootContainerElement =
-    container.nodeType === COMMENT_NODE ? container.parentNode : container;
-  listenToAllSupportedEvents(rootContainerElement);
+    // Initial mount should not be batched.
+    flushSync(() => {
+      updateContainer(initialChildren, root, parentComponent, callback);
+    });
 
-  return root;
+    return root;
+  }
 }
 
 function warnOnInvalidCallback(callback: mixed, callerName: string): void {
@@ -164,39 +212,30 @@ function legacyRenderSubtreeIntoContainer(
     warnOnInvalidCallback(callback === undefined ? null : callback, 'render');
   }
 
-  let root = container._reactRootContainer;
-  let fiberRoot: FiberRoot;
-  if (!root) {
+  const maybeRoot = container._reactRootContainer;
+  let root: FiberRoot;
+  if (!maybeRoot) {
     // Initial mount
-    root = container._reactRootContainer = legacyCreateRootFromDOMContainer(
+    root = legacyCreateRootFromDOMContainer(
       container,
+      children,
+      parentComponent,
+      callback,
       forceHydrate,
     );
-    fiberRoot = root;
-    if (typeof callback === 'function') {
-      const originalCallback = callback;
-      callback = function() {
-        const instance = getPublicRootInstance(fiberRoot);
-        originalCallback.call(instance);
-      };
-    }
-    // Initial mount should not be batched.
-    flushSync(() => {
-      updateContainer(children, fiberRoot, parentComponent, callback);
-    });
   } else {
-    fiberRoot = root;
+    root = maybeRoot;
     if (typeof callback === 'function') {
       const originalCallback = callback;
       callback = function() {
-        const instance = getPublicRootInstance(fiberRoot);
+        const instance = getPublicRootInstance(root);
         originalCallback.call(instance);
       };
     }
     // Update
-    updateContainer(children, fiberRoot, parentComponent, callback);
+    updateContainer(children, root, parentComponent, callback);
   }
-  return getPublicRootInstance(fiberRoot);
+  return getPublicRootInstance(root);
 }
 
 export function findDOMNode(
diff --git a/packages/react-dom/src/client/ReactDOMRoot.js b/packages/react-dom/src/client/ReactDOMRoot.js
index d71de3bb0c26d..6e7192ae10453 100644
--- a/packages/react-dom/src/client/ReactDOMRoot.js
+++ b/packages/react-dom/src/client/ReactDOMRoot.js
@@ -225,7 +225,6 @@ export function createRoot(
   const root = createContainer(
     container,
     ConcurrentRoot,
-    false,
     null,
     isStrictMode,
     concurrentUpdatesByDefaultOverride,
@@ -302,6 +301,7 @@ export function hydrateRoot(
 
   const root = createHydrationContainer(
     initialChildren,
+    null,
     container,
     ConcurrentRoot,
     hydrationCallbacks,
diff --git a/packages/react-native-renderer/src/ReactFabric.js b/packages/react-native-renderer/src/ReactFabric.js
index 127b20fd5dacf..e08a98653fb6c 100644
--- a/packages/react-native-renderer/src/ReactFabric.js
+++ b/packages/react-native-renderer/src/ReactFabric.js
@@ -215,7 +215,6 @@ function render(
     root = createContainer(
       containerTag,
       concurrentRoot ? ConcurrentRoot : LegacyRoot,
-      false,
       null,
       false,
       null,
diff --git a/packages/react-native-renderer/src/ReactNativeRenderer.js b/packages/react-native-renderer/src/ReactNativeRenderer.js
index 1dca2cd7a1d93..e751195dda00a 100644
--- a/packages/react-native-renderer/src/ReactNativeRenderer.js
+++ b/packages/react-native-renderer/src/ReactNativeRenderer.js
@@ -211,7 +211,6 @@ function render(
     root = createContainer(
       containerTag,
       LegacyRoot,
-      false,
       null,
       false,
       null,
diff --git a/packages/react-noop-renderer/src/createReactNoop.js b/packages/react-noop-renderer/src/createReactNoop.js
index e0ba72076a1af..8e4050dcfa336 100644
--- a/packages/react-noop-renderer/src/createReactNoop.js
+++ b/packages/react-noop-renderer/src/createReactNoop.js
@@ -974,7 +974,6 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
         root = NoopRenderer.createContainer(
           container,
           tag,
-          false,
           null,
           null,
           false,
@@ -996,7 +995,6 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
       const fiberRoot = NoopRenderer.createContainer(
         container,
         ConcurrentRoot,
-        false,
         null,
         null,
         false,
@@ -1029,7 +1027,6 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
       const fiberRoot = NoopRenderer.createContainer(
         container,
         LegacyRoot,
-        false,
         null,
         null,
         false,
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.new.js b/packages/react-reconciler/src/ReactFiberReconciler.new.js
index 8607b227e9b40..67eb7f7b232e5 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.new.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.new.js
@@ -245,9 +245,6 @@ function findHostInstanceWithWarning(
 export function createContainer(
   containerInfo: Container,
   tag: RootTag,
-  // TODO: We can remove hydration-specific stuff from createContainer once
-  // we delete legacy mode. The new root API uses createHydrationContainer.
-  hydrate: boolean,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
   isStrictMode: boolean,
   concurrentUpdatesByDefaultOverride: null | boolean,
@@ -255,6 +252,7 @@ export function createContainer(
   onRecoverableError: (error: mixed) => void,
   transitionCallbacks: null | TransitionTracingCallbacks,
 ): OpaqueRoot {
+  const hydrate = false;
   return createFiberRoot(
     containerInfo,
     tag,
@@ -270,6 +268,8 @@ export function createContainer(
 
 export function createHydrationContainer(
   initialChildren: ReactNodeList,
+  // TODO: Remove `callback` when we delete legacy mode.
+  callback: ?Function,
   containerInfo: Container,
   tag: RootTag,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
@@ -305,6 +305,8 @@ export function createHydrationContainer(
   // Caution: React DevTools currently depends on this property
   // being called "element".
   update.payload = {element: initialChildren};
+  update.callback =
+    callback !== undefined && callback !== null ? callback : null;
   enqueueUpdate(current, update, lane);
   scheduleInitialHydrationOnRoot(root, lane, eventTime);
 
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.old.js b/packages/react-reconciler/src/ReactFiberReconciler.old.js
index 4970b685b1c1a..378035e95e661 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.old.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.old.js
@@ -245,9 +245,6 @@ function findHostInstanceWithWarning(
 export function createContainer(
   containerInfo: Container,
   tag: RootTag,
-  // TODO: We can remove hydration-specific stuff from createContainer once
-  // we delete legacy mode. The new root API uses createHydrationContainer.
-  hydrate: boolean,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
   isStrictMode: boolean,
   concurrentUpdatesByDefaultOverride: null | boolean,
@@ -255,6 +252,7 @@ export function createContainer(
   onRecoverableError: (error: mixed) => void,
   transitionCallbacks: null | TransitionTracingCallbacks,
 ): OpaqueRoot {
+  const hydrate = false;
   return createFiberRoot(
     containerInfo,
     tag,
@@ -270,6 +268,8 @@ export function createContainer(
 
 export function createHydrationContainer(
   initialChildren: ReactNodeList,
+  // TODO: Remove `callback` when we delete legacy mode.
+  callback: ?Function,
   containerInfo: Container,
   tag: RootTag,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
@@ -305,6 +305,8 @@ export function createHydrationContainer(
   // Caution: React DevTools currently depends on this property
   // being called "element".
   update.payload = {element: initialChildren};
+  update.callback =
+    callback !== undefined && callback !== null ? callback : null;
   enqueueUpdate(current, update, lane);
   scheduleInitialHydrationOnRoot(root, lane, eventTime);
 
diff --git a/packages/react-reconciler/src/__tests__/ReactFiberHostContext-test.internal.js b/packages/react-reconciler/src/__tests__/ReactFiberHostContext-test.internal.js
index d0c3d5b236ea4..82e23de9965da 100644
--- a/packages/react-reconciler/src/__tests__/ReactFiberHostContext-test.internal.js
+++ b/packages/react-reconciler/src/__tests__/ReactFiberHostContext-test.internal.js
@@ -72,7 +72,6 @@ describe('ReactFiberHostContext', () => {
     const container = Renderer.createContainer(
       /* root: */ null,
       ConcurrentRoot,
-      false,
       null,
       false,
       '',
@@ -136,7 +135,6 @@ describe('ReactFiberHostContext', () => {
     const container = Renderer.createContainer(
       rootContext,
       ConcurrentRoot,
-      false,
       null,
       false,
       '',
diff --git a/packages/react-test-renderer/src/ReactTestRenderer.js b/packages/react-test-renderer/src/ReactTestRenderer.js
index e850086439a67..76911d701de79 100644
--- a/packages/react-test-renderer/src/ReactTestRenderer.js
+++ b/packages/react-test-renderer/src/ReactTestRenderer.js
@@ -473,7 +473,6 @@ function create(element: React$Element<any>, options: TestRendererOptions) {
   let root: FiberRoot | null = createContainer(
     container,
     isConcurrent ? ConcurrentRoot : LegacyRoot,
-    false,
     null,
     isStrictMode,
     concurrentUpdatesByDefault,