diff --git a/packages/react-dom/src/__tests__/ReactDOMRoot-test.js b/packages/react-dom/src/__tests__/ReactDOMRoot-test.js
index 25a560c5c32af..5ffdb38131552 100644
--- a/packages/react-dom/src/__tests__/ReactDOMRoot-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMRoot-test.js
@@ -13,6 +13,7 @@ let React = require('react');
 let ReactDOM = require('react-dom');
 let ReactDOMServer = require('react-dom/server');
 let Scheduler = require('scheduler');
+let act;
 
 describe('ReactDOMRoot', () => {
   let container;
@@ -24,6 +25,7 @@ describe('ReactDOMRoot', () => {
     ReactDOM = require('react-dom');
     ReactDOMServer = require('react-dom/server');
     Scheduler = require('scheduler');
+    act = require('react-dom/test-utils').unstable_concurrentAct;
   });
 
   if (!__EXPERIMENTAL__) {
@@ -316,4 +318,37 @@ describe('ReactDOMRoot', () => {
       {withoutStack: true},
     );
   });
+
+  // @gate experimental
+  it('opts-in to concurrent default updates', async () => {
+    const root = ReactDOM.unstable_createRoot(container, {
+      unstable_concurrentUpdatesByDefault: true,
+    });
+
+    function Foo({value}) {
+      Scheduler.unstable_yieldValue(value);
+      return <div>{value}</div>;
+    }
+
+    await act(async () => {
+      root.render(<Foo value="a" />);
+    });
+
+    expect(container.textContent).toEqual('a');
+
+    await act(async () => {
+      root.render(<Foo value="b" />);
+
+      expect(Scheduler).toHaveYielded(['a']);
+      expect(container.textContent).toEqual('a');
+
+      expect(Scheduler).toFlushAndYieldThrough(['b']);
+      if (gate(flags => flags.allowConcurrentByDefault)) {
+        expect(container.textContent).toEqual('a');
+      } else {
+        expect(container.textContent).toEqual('b');
+      }
+    });
+    expect(container.textContent).toEqual('b');
+  });
 });
diff --git a/packages/react-dom/src/client/ReactDOMRoot.js b/packages/react-dom/src/client/ReactDOMRoot.js
index 56532d5d67488..0a78f61ad0977 100644
--- a/packages/react-dom/src/client/ReactDOMRoot.js
+++ b/packages/react-dom/src/client/ReactDOMRoot.js
@@ -28,6 +28,7 @@ export type RootOptions = {
     ...
   },
   unstable_strictModeLevel?: number,
+  unstable_concurrentUpdatesByDefault?: boolean,
   ...
 };
 
@@ -52,6 +53,7 @@ import {
 } from 'react-reconciler/src/ReactFiberReconciler';
 import invariant from 'shared/invariant';
 import {ConcurrentRoot, LegacyRoot} from 'react-reconciler/src/ReactRootTags';
+import {allowConcurrentByDefault} from 'shared/ReactFeatureFlags';
 
 function ReactDOMRoot(container: Container, options: void | RootOptions) {
   this._internalRoot = createRootImpl(container, ConcurrentRoot, options);
@@ -126,12 +128,21 @@ function createRootImpl(
       ? options.unstable_strictModeLevel
       : null;
 
+  let concurrentUpdatesByDefaultOverride = null;
+  if (allowConcurrentByDefault) {
+    concurrentUpdatesByDefaultOverride =
+      options != null && options.unstable_concurrentUpdatesByDefault != null
+        ? options.unstable_concurrentUpdatesByDefault
+        : null;
+  }
+
   const root = createContainer(
     container,
     tag,
     hydrate,
     hydrationCallbacks,
     strictModeLevelOverride,
+    concurrentUpdatesByDefaultOverride,
   );
   markContainerAsRoot(root.current, container);
 
diff --git a/packages/react-native-renderer/src/ReactFabric.js b/packages/react-native-renderer/src/ReactFabric.js
index 9f1f44f51cf49..b319cb22c7b7b 100644
--- a/packages/react-native-renderer/src/ReactFabric.js
+++ b/packages/react-native-renderer/src/ReactFabric.js
@@ -207,7 +207,7 @@ function render(
   if (!root) {
     // TODO (bvaughn): If we decide to keep the wrapper component,
     // We could create a wrapper for containerTag as well to reduce special casing.
-    root = createContainer(containerTag, LegacyRoot, false, null, null);
+    root = createContainer(containerTag, LegacyRoot, false, null, null, null);
     roots.set(containerTag, root);
   }
   updateContainer(element, root, null, callback);
diff --git a/packages/react-native-renderer/src/ReactNativeRenderer.js b/packages/react-native-renderer/src/ReactNativeRenderer.js
index de42c6e41a58c..4f73377320378 100644
--- a/packages/react-native-renderer/src/ReactNativeRenderer.js
+++ b/packages/react-native-renderer/src/ReactNativeRenderer.js
@@ -203,7 +203,7 @@ function render(
   if (!root) {
     // TODO (bvaughn): If we decide to keep the wrapper component,
     // We could create a wrapper for containerTag as well to reduce special casing.
-    root = createContainer(containerTag, LegacyRoot, false, null, null);
+    root = createContainer(containerTag, LegacyRoot, false, null, null, null);
     roots.set(containerTag, root);
   }
   updateContainer(element, root, null, callback);
diff --git a/packages/react-reconciler/src/ReactFiber.new.js b/packages/react-reconciler/src/ReactFiber.new.js
index 95a5340b18730..c883079028a53 100644
--- a/packages/react-reconciler/src/ReactFiber.new.js
+++ b/packages/react-reconciler/src/ReactFiber.new.js
@@ -24,6 +24,8 @@ import {
   enableStrictEffects,
   enableProfilerTimer,
   enableScopeAPI,
+  enableSyncDefaultUpdates,
+  allowConcurrentByDefault,
 } from 'shared/ReactFeatureFlags';
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
 import {ConcurrentRoot} from './ReactRootTags';
@@ -68,6 +70,7 @@ import {
   ProfileMode,
   StrictLegacyMode,
   StrictEffectsMode,
+  ConcurrentUpdatesByDefaultMode,
 } from './ReactTypeOfMode';
 import {
   REACT_FORWARD_REF_TYPE,
@@ -420,6 +423,7 @@ export function resetWorkInProgress(workInProgress: Fiber, renderLanes: Lanes) {
 export function createHostRootFiber(
   tag: RootTag,
   strictModeLevelOverride: null | number,
+  concurrentUpdatesByDefaultOverride: null | boolean,
 ): Fiber {
   let mode;
   if (tag === ConcurrentRoot) {
@@ -440,6 +444,15 @@ export function createHostRootFiber(
         mode |= StrictLegacyMode;
       }
     }
+    if (
+      // We only use this flag for our repo tests to check both behaviors.
+      // TODO: Flip this flag and rename it something like "forceConcurrentByDefaultForTesting"
+      !enableSyncDefaultUpdates ||
+      // Only for internal experiments.
+      (allowConcurrentByDefault && concurrentUpdatesByDefaultOverride)
+    ) {
+      mode |= ConcurrentUpdatesByDefaultMode;
+    }
   } else {
     mode = NoMode;
   }
diff --git a/packages/react-reconciler/src/ReactFiber.old.js b/packages/react-reconciler/src/ReactFiber.old.js
index 159557e86a166..e33c0e7cd835f 100644
--- a/packages/react-reconciler/src/ReactFiber.old.js
+++ b/packages/react-reconciler/src/ReactFiber.old.js
@@ -24,6 +24,8 @@ import {
   enableStrictEffects,
   enableProfilerTimer,
   enableScopeAPI,
+  enableSyncDefaultUpdates,
+  allowConcurrentByDefault,
 } from 'shared/ReactFeatureFlags';
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
 import {ConcurrentRoot} from './ReactRootTags';
@@ -68,6 +70,7 @@ import {
   ProfileMode,
   StrictLegacyMode,
   StrictEffectsMode,
+  ConcurrentUpdatesByDefaultMode,
 } from './ReactTypeOfMode';
 import {
   REACT_FORWARD_REF_TYPE,
@@ -420,6 +423,7 @@ export function resetWorkInProgress(workInProgress: Fiber, renderLanes: Lanes) {
 export function createHostRootFiber(
   tag: RootTag,
   strictModeLevelOverride: null | number,
+  concurrentUpdatesByDefaultOverride: null | boolean,
 ): Fiber {
   let mode;
   if (tag === ConcurrentRoot) {
@@ -440,6 +444,15 @@ export function createHostRootFiber(
         mode |= StrictLegacyMode;
       }
     }
+    if (
+      // We only use this flag for our repo tests to check both behaviors.
+      // TODO: Flip this flag and rename it something like "forceConcurrentByDefaultForTesting"
+      !enableSyncDefaultUpdates ||
+      // Only for internal experiments.
+      (allowConcurrentByDefault && concurrentUpdatesByDefaultOverride)
+    ) {
+      mode |= ConcurrentUpdatesByDefaultMode;
+    }
   } else {
     mode = NoMode;
   }
diff --git a/packages/react-reconciler/src/ReactFiberLane.new.js b/packages/react-reconciler/src/ReactFiberLane.new.js
index 64a3bc312cda2..cdf5543c36e6f 100644
--- a/packages/react-reconciler/src/ReactFiberLane.new.js
+++ b/packages/react-reconciler/src/ReactFiberLane.new.js
@@ -39,9 +39,10 @@ import {
   enableCache,
   enableSchedulingProfiler,
   enableUpdaterTracking,
-  enableSyncDefaultUpdates,
+  allowConcurrentByDefault,
 } from 'shared/ReactFeatureFlags';
 import {isDevToolsPresent} from './ReactFiberDevToolsHook.new';
+import {ConcurrentUpdatesByDefaultMode, NoMode} from './ReactTypeOfMode';
 
 export const SyncLanePriority: LanePriority = 12;
 
@@ -318,11 +319,12 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
   }
 
   if (
-    // TODO: Check for root override, once that lands
-    enableSyncDefaultUpdates &&
-    (nextLanes & InputContinuousLane) !== NoLanes
+    allowConcurrentByDefault &&
+    (root.current.mode & ConcurrentUpdatesByDefaultMode) !== NoMode
   ) {
-    // When updates are sync by default, we entangle continous priority updates
+    // Do nothing, use the lanes as they were assigned.
+  } else if ((nextLanes & InputContinuousLane) !== NoLanes) {
+    // When updates are sync by default, we entangle continuous priority updates
     // and default updates, so they render in the same batch. The only reason
     // they use separate lanes is because continuous updates should interrupt
     // transitions, but default updates should not.
@@ -527,17 +529,21 @@ export function shouldTimeSlice(root: FiberRoot, lanes: Lanes) {
     // finish rendering without yielding execution.
     return false;
   }
-  if (enableSyncDefaultUpdates) {
-    const SyncDefaultLanes =
-      InputContinuousHydrationLane |
-      InputContinuousLane |
-      DefaultHydrationLane |
-      DefaultLane;
-    // TODO: Check for root override, once that lands
-    return (lanes & SyncDefaultLanes) === NoLanes;
-  } else {
+
+  if (
+    allowConcurrentByDefault &&
+    (root.current.mode & ConcurrentUpdatesByDefaultMode) !== NoMode
+  ) {
+    // Concurrent updates by default always use time slicing.
     return true;
   }
+
+  const SyncDefaultLanes =
+    InputContinuousHydrationLane |
+    InputContinuousLane |
+    DefaultHydrationLane |
+    DefaultLane;
+  return (lanes & SyncDefaultLanes) === NoLanes;
 }
 
 export function isTransitionLane(lane: Lane) {
diff --git a/packages/react-reconciler/src/ReactFiberLane.old.js b/packages/react-reconciler/src/ReactFiberLane.old.js
index 69c811a80e9b5..6ab7163cbb927 100644
--- a/packages/react-reconciler/src/ReactFiberLane.old.js
+++ b/packages/react-reconciler/src/ReactFiberLane.old.js
@@ -39,9 +39,10 @@ import {
   enableCache,
   enableSchedulingProfiler,
   enableUpdaterTracking,
-  enableSyncDefaultUpdates,
+  allowConcurrentByDefault,
 } from 'shared/ReactFeatureFlags';
 import {isDevToolsPresent} from './ReactFiberDevToolsHook.old';
+import {ConcurrentUpdatesByDefaultMode, NoMode} from './ReactTypeOfMode';
 
 export const SyncLanePriority: LanePriority = 12;
 
@@ -318,11 +319,12 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
   }
 
   if (
-    // TODO: Check for root override, once that lands
-    enableSyncDefaultUpdates &&
-    (nextLanes & InputContinuousLane) !== NoLanes
+    allowConcurrentByDefault &&
+    (root.current.mode & ConcurrentUpdatesByDefaultMode) !== NoMode
   ) {
-    // When updates are sync by default, we entangle continous priority updates
+    // Do nothing, use the lanes as they were assigned.
+  } else if ((nextLanes & InputContinuousLane) !== NoLanes) {
+    // When updates are sync by default, we entangle continuous priority updates
     // and default updates, so they render in the same batch. The only reason
     // they use separate lanes is because continuous updates should interrupt
     // transitions, but default updates should not.
@@ -527,17 +529,21 @@ export function shouldTimeSlice(root: FiberRoot, lanes: Lanes) {
     // finish rendering without yielding execution.
     return false;
   }
-  if (enableSyncDefaultUpdates) {
-    const SyncDefaultLanes =
-      InputContinuousHydrationLane |
-      InputContinuousLane |
-      DefaultHydrationLane |
-      DefaultLane;
-    // TODO: Check for root override, once that lands
-    return (lanes & SyncDefaultLanes) === NoLanes;
-  } else {
+
+  if (
+    allowConcurrentByDefault &&
+    (root.current.mode & ConcurrentUpdatesByDefaultMode) !== NoMode
+  ) {
+    // Concurrent updates by default always use time slicing.
     return true;
   }
+
+  const SyncDefaultLanes =
+    InputContinuousHydrationLane |
+    InputContinuousLane |
+    DefaultHydrationLane |
+    DefaultLane;
+  return (lanes & SyncDefaultLanes) === NoLanes;
 }
 
 export function isTransitionLane(lane: Lane) {
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.new.js b/packages/react-reconciler/src/ReactFiberReconciler.new.js
index f6318f0a03052..f53a24e86c60b 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.new.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.new.js
@@ -249,6 +249,7 @@ export function createContainer(
   hydrate: boolean,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
   strictModeLevelOverride: null | number,
+  concurrentUpdatesByDefaultOverride: null | boolean,
 ): OpaqueRoot {
   return createFiberRoot(
     containerInfo,
@@ -256,6 +257,7 @@ export function createContainer(
     hydrate,
     hydrationCallbacks,
     strictModeLevelOverride,
+    concurrentUpdatesByDefaultOverride,
   );
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.old.js b/packages/react-reconciler/src/ReactFiberReconciler.old.js
index 68a6ed619bbda..00c12b3406df8 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.old.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.old.js
@@ -249,6 +249,7 @@ export function createContainer(
   hydrate: boolean,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
   strictModeLevelOverride: null | number,
+  concurrentUpdatesByDefaultOverride: null | boolean,
 ): OpaqueRoot {
   return createFiberRoot(
     containerInfo,
@@ -256,6 +257,7 @@ export function createContainer(
     hydrate,
     hydrationCallbacks,
     strictModeLevelOverride,
+    concurrentUpdatesByDefaultOverride,
   );
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberRoot.new.js b/packages/react-reconciler/src/ReactFiberRoot.new.js
index e82262b5737e7..d2efcc5851d49 100644
--- a/packages/react-reconciler/src/ReactFiberRoot.new.js
+++ b/packages/react-reconciler/src/ReactFiberRoot.new.js
@@ -99,6 +99,7 @@ export function createFiberRoot(
   hydrate: boolean,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
   strictModeLevelOverride: null | number,
+  concurrentUpdatesByDefaultOverride: null | boolean,
 ): FiberRoot {
   const root: FiberRoot = (new FiberRootNode(containerInfo, tag, hydrate): any);
   if (enableSuspenseCallback) {
@@ -107,7 +108,11 @@ export function createFiberRoot(
 
   // Cyclic construction. This cheats the type system right now because
   // stateNode is any.
-  const uninitializedFiber = createHostRootFiber(tag, strictModeLevelOverride);
+  const uninitializedFiber = createHostRootFiber(
+    tag,
+    strictModeLevelOverride,
+    concurrentUpdatesByDefaultOverride,
+  );
   root.current = uninitializedFiber;
   uninitializedFiber.stateNode = root;
 
diff --git a/packages/react-reconciler/src/ReactFiberRoot.old.js b/packages/react-reconciler/src/ReactFiberRoot.old.js
index 20c3cdaa6cbff..68d329492cf1b 100644
--- a/packages/react-reconciler/src/ReactFiberRoot.old.js
+++ b/packages/react-reconciler/src/ReactFiberRoot.old.js
@@ -99,6 +99,7 @@ export function createFiberRoot(
   hydrate: boolean,
   hydrationCallbacks: null | SuspenseHydrationCallbacks,
   strictModeLevelOverride: null | number,
+  concurrentUpdatesByDefaultOverride: null | boolean,
 ): FiberRoot {
   const root: FiberRoot = (new FiberRootNode(containerInfo, tag, hydrate): any);
   if (enableSuspenseCallback) {
@@ -107,7 +108,11 @@ export function createFiberRoot(
 
   // Cyclic construction. This cheats the type system right now because
   // stateNode is any.
-  const uninitializedFiber = createHostRootFiber(tag, strictModeLevelOverride);
+  const uninitializedFiber = createHostRootFiber(
+    tag,
+    strictModeLevelOverride,
+    concurrentUpdatesByDefaultOverride,
+  );
   root.current = uninitializedFiber;
   uninitializedFiber.stateNode = root;
 
diff --git a/packages/react-reconciler/src/ReactTypeOfMode.js b/packages/react-reconciler/src/ReactTypeOfMode.js
index 466363fabd4e8..f16d2c51b76c0 100644
--- a/packages/react-reconciler/src/ReactTypeOfMode.js
+++ b/packages/react-reconciler/src/ReactTypeOfMode.js
@@ -9,10 +9,11 @@
 
 export type TypeOfMode = number;
 
-export const NoMode = /*            */ 0b000000;
+export const NoMode = /*                         */ 0b000000;
 // TODO: Remove ConcurrentMode by reading from the root tag instead
-export const ConcurrentMode = /*    */ 0b000001;
-export const ProfileMode = /*       */ 0b000010;
-export const DebugTracingMode = /*  */ 0b000100;
-export const StrictLegacyMode = /*  */ 0b001000;
-export const StrictEffectsMode = /* */ 0b010000;
+export const ConcurrentMode = /*                 */ 0b000001;
+export const ProfileMode = /*                    */ 0b000010;
+export const DebugTracingMode = /*               */ 0b000100;
+export const StrictLegacyMode = /*               */ 0b001000;
+export const StrictEffectsMode = /*              */ 0b010000;
+export const ConcurrentUpdatesByDefaultMode = /* */ 0b100000;
diff --git a/packages/react-test-renderer/src/ReactTestRenderer.js b/packages/react-test-renderer/src/ReactTestRenderer.js
index 5c74c6e9e310c..7c64a16936005 100644
--- a/packages/react-test-renderer/src/ReactTestRenderer.js
+++ b/packages/react-test-renderer/src/ReactTestRenderer.js
@@ -51,6 +51,7 @@ import enqueueTask from 'shared/enqueueTask';
 
 import {getPublicInstance} from './ReactTestHostConfig';
 import {ConcurrentRoot, LegacyRoot} from 'react-reconciler/src/ReactRootTags';
+import {allowConcurrentByDefault} from 'shared/ReactFeatureFlags';
 
 const {IsSomeRendererActing} = ReactSharedInternals;
 
@@ -58,6 +59,7 @@ type TestRendererOptions = {
   createNodeMock: (element: React$Element<any>) => any,
   unstable_isConcurrent: boolean,
   unstable_strictModeLevel: number,
+  unstable_concurrentUpdatesByDefault: boolean,
   ...
 };
 
@@ -436,6 +438,7 @@ function create(element: React$Element<any>, options: TestRendererOptions) {
   let createNodeMock = defaultTestOptions.createNodeMock;
   let isConcurrent = false;
   let strictModeLevel = null;
+  let concurrentUpdatesByDefault = null;
   if (typeof options === 'object' && options !== null) {
     if (typeof options.createNodeMock === 'function') {
       createNodeMock = options.createNodeMock;
@@ -446,6 +449,12 @@ function create(element: React$Element<any>, options: TestRendererOptions) {
     if (options.unstable_strictModeLevel !== undefined) {
       strictModeLevel = options.unstable_strictModeLevel;
     }
+    if (allowConcurrentByDefault) {
+      if (options.unstable_concurrentUpdatesByDefault !== undefined) {
+        concurrentUpdatesByDefault =
+          options.unstable_concurrentUpdatesByDefault;
+      }
+    }
   }
   let container = {
     children: [],
@@ -458,6 +467,7 @@ function create(element: React$Element<any>, options: TestRendererOptions) {
     false,
     null,
     strictModeLevel,
+    concurrentUpdatesByDefault,
   );
   invariant(root != null, 'something went wrong');
   updateContainer(element, root, null, null);
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index 757d9ab28b784..ede084ff74ace 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -169,3 +169,5 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 
 export const enableSyncDefaultUpdates = true;
+
+export const allowConcurrentByDefault = false;
diff --git a/packages/shared/forks/ReactFeatureFlags.native-fb.js b/packages/shared/forks/ReactFeatureFlags.native-fb.js
index ea9224d5a9445..a2b9cc8446c4c 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-fb.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-fb.js
@@ -60,6 +60,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = false;
+export const allowConcurrentByDefault = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.native-oss.js b/packages/shared/forks/ReactFeatureFlags.native-oss.js
index 251bb310c1fb6..eb3a55187e7d5 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-oss.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-oss.js
@@ -59,6 +59,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = true;
+export const allowConcurrentByDefault = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
index aeee968922f12..1a8a75a4c60fb 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
@@ -59,6 +59,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = true;
+export const allowConcurrentByDefault = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js
index 88f2287cd81b4..727e8764fc126 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.native.js
@@ -59,6 +59,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = false;
+export const allowConcurrentByDefault = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
index 5d43505fa1279..688d30ee9fe2b 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
@@ -59,6 +59,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = false;
+export const allowConcurrentByDefault = true;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.testing.js b/packages/shared/forks/ReactFeatureFlags.testing.js
index 0edbfad4ab170..d593aa3f9cee5 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.js
@@ -59,6 +59,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = true;
+export const allowConcurrentByDefault = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.testing.www.js b/packages/shared/forks/ReactFeatureFlags.testing.www.js
index 98dbae4505e08..01f184c1e4584 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.www.js
@@ -59,6 +59,7 @@ export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = true;
+export const allowConcurrentByDefault = true;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
diff --git a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
index d0fc8e6177067..b8c7a56ce720c 100644
--- a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
+++ b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
@@ -60,3 +60,4 @@ export const enableProfilerNestedUpdateScheduledHook = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableLazyContextPropagation = __VARIANT__;
 export const enableSyncDefaultUpdates = __VARIANT__;
+export const allowConcurrentByDefault = true;
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index afae32b4082fb..1d9c88cc8fcfb 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -93,6 +93,8 @@ export const enableNewReconciler = __VARIANT__;
 
 export const enableRecursiveCommitTraversal = false;
 
+export const allowConcurrentByDefault = true;
+
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;