diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationCheckbox-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationCheckbox-test.js
index 606ec88b8989..178ed7982a44 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationCheckbox-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationCheckbox-test.js
@@ -15,7 +15,7 @@ const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegratio
 const {disableInputAttributeSyncing} = require('shared/ReactFeatureFlags');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -23,13 +23,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationClassContextType-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationClassContextType-test.js
index dda475b7ced3..2df2d66b9b9f 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationClassContextType-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationClassContextType-test.js
@@ -13,7 +13,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -21,13 +21,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationFragment-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationFragment-test.js
index 680f283b6dbf..8e8fc2aa8fe2 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationFragment-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationFragment-test.js
@@ -13,7 +13,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -21,13 +21,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationInput-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationInput-test.js
index afbbf28a41ec..54780dae52cd 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationInput-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationInput-test.js
@@ -15,7 +15,7 @@ const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegratio
 const {disableInputAttributeSyncing} = require('shared/ReactFeatureFlags');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -23,13 +23,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationModes-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationModes-test.js
index e551a72b9ace..99cf33b821f1 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationModes-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationModes-test.js
@@ -13,7 +13,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -21,13 +21,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js
index eb201c17b484..76612f510d7d 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationReconnecting-test.js
@@ -13,30 +13,30 @@ const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegratio
 
 let React;
 let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
-function initModules() {
-  // Reset warning cache.
-  jest.resetModules();
-
-  React = require('react');
-  ReactDOM = require('react-dom');
-  ReactDOMServer = require('react-dom/server');
-  ReactTestUtils = require('react-dom/test-utils');
-
-  // Make them available to the helpers.
-  return {
-    ReactDOM,
-    ReactDOMServer,
-    ReactTestUtils,
-  };
-}
-
-const {resetModules, expectMarkupMismatch, expectMarkupMatch} =
-  ReactDOMServerIntegrationUtils(initModules);
-
 describe('ReactDOMServerIntegration', () => {
+  function initModules() {
+    // Reset warning cache.
+    jest.resetModules();
+
+    React = require('react');
+    ReactDOMClient = require('react-dom/client');
+    ReactDOMServer = require('react-dom/server');
+    ReactTestUtils = require('react-dom/test-utils');
+
+    // Make them available to the helpers.
+    return {
+      ReactDOMClient,
+      ReactDOMServer,
+      ReactTestUtils,
+    };
+  }
+
+  const {resetModules, expectMarkupMismatch, expectMarkupMatch} =
+    ReactDOMServerIntegrationUtils(initModules);
   beforeEach(() => {
     resetModules();
   });
@@ -123,8 +123,8 @@ describe('ReactDOMServerIntegration', () => {
       it('should error reconnecting different attribute values', () =>
         expectMarkupMismatch(<div id="foo" />, <div id="bar" />));
 
-      it('can explicitly ignore errors reconnecting different element types of children', () =>
-        expectMarkupMatch(
+      it('should error reconnecting different element types of children', () =>
+        expectMarkupMismatch(
           <div>
             <div />
           </div>,
@@ -354,8 +354,8 @@ describe('ReactDOMServerIntegration', () => {
           <div>{''}</div>,
         ));
 
-      it('can explicitly ignore reconnecting more children', () =>
-        expectMarkupMatch(
+      it('can not ignore reconnecting more children', () =>
+        expectMarkupMismatch(
           <div>
             <div />
           </div>,
@@ -365,8 +365,8 @@ describe('ReactDOMServerIntegration', () => {
           </div>,
         ));
 
-      it('can explicitly ignore reconnecting fewer children', () =>
-        expectMarkupMatch(
+      it('can not ignore reconnecting fewer children', () =>
+        expectMarkupMismatch(
           <div>
             <div />
             <div />
@@ -376,8 +376,8 @@ describe('ReactDOMServerIntegration', () => {
           </div>,
         ));
 
-      it('can explicitly ignore reconnecting reordered children', () =>
-        expectMarkupMatch(
+      it('can not ignore reconnecting reordered children', () =>
+        expectMarkupMismatch(
           <div suppressHydrationWarning={true}>
             <div />
             <span />
@@ -456,3 +456,73 @@ describe('ReactDOMServerIntegration', () => {
       ));
   });
 });
+
+describe('ReactDOMServerIntegration (legacy)', () => {
+  function initModules() {
+    // Reset warning cache.
+    jest.resetModules();
+
+    React = require('react');
+    ReactDOM = require('react-dom');
+    ReactDOMServer = require('react-dom/server');
+    ReactTestUtils = require('react-dom/test-utils');
+
+    // Make them available to the helpers.
+    return {
+      ReactDOM,
+      ReactDOMServer,
+      ReactTestUtils,
+    };
+  }
+
+  const {resetModules, expectMarkupMatch} =
+    ReactDOMServerIntegrationUtils(initModules);
+
+  beforeEach(() => {
+    resetModules();
+  });
+
+  it('legacy mode can explicitly ignore errors reconnecting different element types of children', () =>
+    expectMarkupMatch(
+      <div>
+        <div />
+      </div>,
+      <div suppressHydrationWarning={true}>
+        <span />
+      </div>,
+    ));
+
+  it('legacy mode can explicitly ignore reconnecting more children', () =>
+    expectMarkupMatch(
+      <div>
+        <div />
+      </div>,
+      <div suppressHydrationWarning={true}>
+        <div />
+        <div />
+      </div>,
+    ));
+
+  it('legacy mode can explicitly ignore reconnecting fewer children', () =>
+    expectMarkupMatch(
+      <div>
+        <div />
+        <div />
+      </div>,
+      <div suppressHydrationWarning={true}>
+        <div />
+      </div>,
+    ));
+
+  it('legacy mode can explicitly ignore reconnecting reordered children', () =>
+    expectMarkupMatch(
+      <div suppressHydrationWarning={true}>
+        <div />
+        <span />
+      </div>,
+      <div suppressHydrationWarning={true}>
+        <span />
+        <div />
+      </div>,
+    ));
+});
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationRefs-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationRefs-test.js
index 76da3e92c82a..e5564d3d9348 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationRefs-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationRefs-test.js
@@ -12,7 +12,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -20,13 +20,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationSpecialTypes-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationSpecialTypes-test.js
index f3a8b869ad81..8ea1c9d53bae 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationSpecialTypes-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationSpecialTypes-test.js
@@ -13,7 +13,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 let forwardRef;
@@ -26,7 +26,7 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
   forwardRef = React.forwardRef;
@@ -44,7 +44,7 @@ function initModules() {
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationTextarea-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationTextarea-test.js
index 697ec7f340d8..dd19385e62c5 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationTextarea-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationTextarea-test.js
@@ -13,7 +13,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -21,13 +21,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationUserInteraction-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationUserInteraction-test.js
index b335b03b01d3..bc5980f23dda 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationUserInteraction-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationUserInteraction-test.js
@@ -12,7 +12,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -20,13 +20,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/ReactDOMserverIntegrationProgress-test.js b/packages/react-dom/src/__tests__/ReactDOMserverIntegrationProgress-test.js
index b949e0ab522f..cf51eff4aced 100644
--- a/packages/react-dom/src/__tests__/ReactDOMserverIntegrationProgress-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMserverIntegrationProgress-test.js
@@ -13,7 +13,7 @@
 const ReactDOMServerIntegrationUtils = require('./utils/ReactDOMServerIntegrationTestUtils');
 
 let React;
-let ReactDOM;
+let ReactDOMClient;
 let ReactDOMServer;
 let ReactTestUtils;
 
@@ -21,13 +21,13 @@ function initModules() {
   // Reset warning cache.
   jest.resetModules();
   React = require('react');
-  ReactDOM = require('react-dom');
+  ReactDOMClient = require('react-dom/client');
   ReactDOMServer = require('react-dom/server');
   ReactTestUtils = require('react-dom/test-utils');
 
   // Make them available to the helpers.
   return {
-    ReactDOM,
+    ReactDOMClient,
     ReactDOMServer,
     ReactTestUtils,
   };
diff --git a/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js b/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js
index 8392b57af03c..e9c4479b028f 100644
--- a/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js
+++ b/packages/react-dom/src/__tests__/utils/ReactDOMServerIntegrationTestUtils.js
@@ -14,11 +14,12 @@ const shouldIgnoreConsoleError = require('../../../../../scripts/jest/shouldIgno
 
 module.exports = function (initModules) {
   let ReactDOM;
+  let ReactDOMClient;
   let ReactDOMServer;
   let act;
 
   function resetModules() {
-    ({ReactDOM, ReactDOMServer} = initModules());
+    ({ReactDOM, ReactDOMClient, ReactDOMServer} = initModules());
     act = require('internal-test-utils').act;
   }
 
@@ -51,11 +52,24 @@ module.exports = function (initModules) {
   async function asyncReactDOMRender(reactElement, domElement, forceHydrate) {
     if (forceHydrate) {
       await act(() => {
-        ReactDOM.hydrate(reactElement, domElement);
+        if (ReactDOMClient) {
+          ReactDOMClient.hydrateRoot(domElement, reactElement, {
+            onRecoverableError: () => {
+              // TODO: assert on recoverable error count.
+            },
+          });
+        } else {
+          ReactDOM.hydrate(reactElement, domElement);
+        }
       });
     } else {
       await act(() => {
-        ReactDOM.render(reactElement, domElement);
+        if (ReactDOMClient) {
+          const root = ReactDOMClient.createRoot(domElement);
+          root.render(reactElement);
+        } else {
+          ReactDOM.render(reactElement, domElement);
+        }
       });
     }
   }
@@ -80,7 +94,11 @@ module.exports = function (initModules) {
       for (let i = 0; i < console.error.mock.calls.length; i++) {
         const args = console.error.mock.calls[i];
         const [format, ...rest] = args;
-        if (!shouldIgnoreConsoleError(format, rest)) {
+        if (
+          !shouldIgnoreConsoleError(format, rest, {
+            TODO_ignoreHydrationErrors: true,
+          })
+        ) {
           filteredWarnings.push(args);
         }
       }
diff --git a/scripts/jest/shouldIgnoreConsoleError.js b/scripts/jest/shouldIgnoreConsoleError.js
index 42aae220debe..79ec7fc2ad7d 100644
--- a/scripts/jest/shouldIgnoreConsoleError.js
+++ b/scripts/jest/shouldIgnoreConsoleError.js
@@ -1,6 +1,10 @@
 'use strict';
 
-module.exports = function shouldIgnoreConsoleError(format, args) {
+module.exports = function shouldIgnoreConsoleError(
+  format,
+  args,
+  {TODO_ignoreHydrationErrors} = {TODO_ignoreHydrationErrors: false}
+) {
   if (__DEV__) {
     if (typeof format === 'string') {
       if (format.indexOf('Error: Uncaught [') === 0) {
@@ -23,6 +27,15 @@ module.exports = function shouldIgnoreConsoleError(format, args) {
         // We haven't finished migrating our tests to use createRoot.
         return true;
       }
+      if (
+        TODO_ignoreHydrationErrors &&
+        format.indexOf(
+          'An error occurred during hydration. The server HTML was replaced with client content in'
+        ) !== -1
+      ) {
+        // This also gets logged by onRecoverableError, so we can ignore it.
+        return true;
+      }
     } else if (
       format != null &&
       typeof format.message === 'string' &&