diff --git a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationHooks-test.js b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationHooks-test.js
index 8c90d593a7..79345cc146 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerIntegrationHooks-test.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerIntegrationHooks-test.js
@@ -1686,7 +1686,9 @@ describe('ReactDOMServerHooks', () => {
           <App />,
         );
 
-        if (gate(flags => flags.new)) {
+        if (
+          gate(flags => flags.new && flags.deferRenderPhaseUpdateToNextBatch)
+        ) {
           expect(() => Scheduler.unstable_flushAll()).toErrorDev([
             'The object passed back from useOpaqueIdentifier is meant to be passed through to attributes only. ' +
               'Do not read the value directly.',
@@ -1730,7 +1732,9 @@ describe('ReactDOMServerHooks', () => {
           <App />,
         );
 
-        if (gate(flags => flags.new)) {
+        if (
+          gate(flags => flags.new && flags.deferRenderPhaseUpdateToNextBatch)
+        ) {
           expect(() => Scheduler.unstable_flushAll()).toErrorDev([
             'The object passed back from useOpaqueIdentifier is meant to be passed through to attributes only. ' +
               'Do not read the value directly.',
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index e2ff1032b8..3c6b2105d0 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -27,6 +27,7 @@ import {
   enableProfilerCommitHooks,
   enableSchedulerTracing,
   warnAboutUnmockedScheduler,
+  deferRenderPhaseUpdateToNextBatch,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -123,6 +124,7 @@ import {
   isSubsetOfLanes,
   mergeLanes,
   removeLanes,
+  pickArbitraryLane,
   hasDiscreteLanes,
   hasUpdatePriority,
   getNextLanes,
@@ -354,6 +356,21 @@ export function requestUpdateLane(
     return getCurrentPriorityLevel() === ImmediateSchedulerPriority
       ? (SyncLane: Lane)
       : (SyncBatchedLane: Lane);
+  } else if (
+    !deferRenderPhaseUpdateToNextBatch &&
+    (executionContext & RenderContext) !== NoContext &&
+    workInProgressRootRenderLanes !== NoLanes
+  ) {
+    // This is a render phase update. These are not officially supported. The
+    // old behavior is to give this the same "thread" (expiration time) as
+    // whatever is currently rendering. So if you call `setState` on a component
+    // that happens later in the same render, it will flush. Ideally, we want to
+    // remove the special case and treat them as if they came from an
+    // interleaved event. Regardless, this pattern is not officially supported.
+    // This behavior is only a fallback. The flag only exists until we can roll
+    // out the setState warnning, since existing code might accidentally rely on
+    // the current behavior.
+    return pickArbitraryLane(workInProgressRootRenderLanes);
   }
 
   // The algorithm for assigning an update to a lane should be stable for all
@@ -542,11 +559,19 @@ function markUpdateLaneFromFiberToRoot(
     markRootUpdated(root, lane);
     if (workInProgressRoot === root) {
       // Received an update to a tree that's in the middle of rendering. Mark
-      // that there is unprocessed work on this root.
-      workInProgressRootUpdatedLanes = mergeLanes(
-        workInProgressRootUpdatedLanes,
-        lane,
-      );
+      // that there was an interleaved update work on this root. Unless the
+      // `deferRenderPhaseUpdateToNextBatch` flag is off and this is a render
+      // phase update. In that case, we don't treat render phase updates as if
+      // they were interleaved, for backwards compat reasons.
+      if (
+        deferRenderPhaseUpdateToNextBatch ||
+        (executionContext & RenderContext) === NoContext
+      ) {
+        workInProgressRootUpdatedLanes = mergeLanes(
+          workInProgressRootUpdatedLanes,
+          lane,
+        );
+      }
       if (workInProgressRootExitStatus === RootSuspendedWithDelay) {
         // The root already suspended with a delay, which means this render
         // definitely won't finish. Since we have a new update, let's mark it as
diff --git a/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.js b/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.js
index 90a327d734..055a2ee5c6 100644
--- a/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactIncrementalUpdates-test.js
@@ -394,7 +394,7 @@ describe('ReactIncrementalUpdates', () => {
     expect(() =>
       expect(Scheduler).toFlushAndYield(
         gate(flags =>
-          flags.new
+          flags.new && flags.deferRenderPhaseUpdateToNextBatch
             ? [
                 'setState updater',
                 // In the new reconciler, updates inside the render phase are
@@ -427,7 +427,7 @@ describe('ReactIncrementalUpdates', () => {
     });
     expect(Scheduler).toFlushAndYield(
       gate(flags =>
-        flags.new
+        flags.new && flags.deferRenderPhaseUpdateToNextBatch
           ? // In the new reconciler, updates inside the render phase are
             // treated as if they came from an event, so the update gets shifted
             // to a subsequent render.
diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index 314b9a2447..b0e85d43d3 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -127,3 +127,11 @@ export const enableModernEventSystem = false;
 
 // Support legacy Primer support on internal FB www
 export const enableLegacyFBSupport = false;
+
+// Updates that occur in the render phase are not officially supported. But when
+// they do occur, in the new reconciler, we defer them to a subsequent render by
+// picking a lane that's not currently rendering. We treat them the same as if
+// they came from an interleaved event. In the old reconciler, we use whatever
+// expiration time is currently rendering. Remove this flag once we have
+// migrated to the new behavior.
+export const deferRenderPhaseUpdateToNextBatch = true;
diff --git a/packages/shared/forks/ReactFeatureFlags.native-fb.js b/packages/shared/forks/ReactFeatureFlags.native-fb.js
index 1d4198c3fb..978c5ff8cd 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-fb.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-fb.js
@@ -46,6 +46,7 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 
 export const enableNewReconciler = false;
+export const deferRenderPhaseUpdateToNextBatch = 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.native-oss.js b/packages/shared/forks/ReactFeatureFlags.native-oss.js
index 6ba9fa00cf..9c00c127f2 100644
--- a/packages/shared/forks/ReactFeatureFlags.native-oss.js
+++ b/packages/shared/forks/ReactFeatureFlags.native-oss.js
@@ -45,6 +45,7 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 
 export const enableNewReconciler = false;
+export const deferRenderPhaseUpdateToNextBatch = 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.test-renderer.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
index 21ed6623e1..93ff1a7fa0 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.js
@@ -45,6 +45,7 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 
 export const enableNewReconciler = false;
+export const deferRenderPhaseUpdateToNextBatch = 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.test-renderer.www.js b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
index 5cd46d0cff..c84df882fb 100644
--- a/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.test-renderer.www.js
@@ -45,6 +45,7 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 
 export const enableNewReconciler = false;
+export const deferRenderPhaseUpdateToNextBatch = 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 41971ca576..2aa405ebe6 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.js
@@ -45,6 +45,7 @@ export const enableLegacyFBSupport = false;
 export const enableFilterEmptyStringAttributesDOM = false;
 
 export const enableNewReconciler = false;
+export const deferRenderPhaseUpdateToNextBatch = 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.www.js b/packages/shared/forks/ReactFeatureFlags.testing.www.js
index 12c09a97e6..3c7ae77f8e 100644
--- a/packages/shared/forks/ReactFeatureFlags.testing.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.testing.www.js
@@ -45,6 +45,7 @@ export const enableLegacyFBSupport = !__EXPERIMENTAL__;
 export const enableFilterEmptyStringAttributesDOM = false;
 
 export const enableNewReconciler = false;
+export const deferRenderPhaseUpdateToNextBatch = 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 766b4ebe7d..a047ec5d72 100644
--- a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
+++ b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
@@ -21,6 +21,16 @@ export const enableModernEventSystem = __VARIANT__;
 export const enableLegacyFBSupport = __VARIANT__;
 export const enableDebugTracing = !__VARIANT__;
 
+// This only has an effect in the new reconciler. But also, the new reconciler
+// is only enabled when __VARIANT__ is true. So this is set to the opposite of
+// __VARIANT__ so that it's `false` when running against the new reconciler.
+// Ideally we would test both against the new reconciler, but until then, we
+// should test the value that is used in www. Which is `false`.
+//
+// Once Lanes has landed in both reconciler forks, we'll get coverage of
+// both branches.
+export const deferRenderPhaseUpdateToNextBatch = !__VARIANT__;
+
 // These are already tested in both modes using the build type dimension,
 // so we don't need to use __VARIANT__ to get extra coverage.
 export const debugRenderPhaseSideEffectsForStrictMode = __DEV__;
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index d21c7b7b69..cc982e8cd5 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -26,6 +26,7 @@ export const {
   enableFilterEmptyStringAttributesDOM,
   enableLegacyFBSupport,
   enableDebugTracing,
+  deferRenderPhaseUpdateToNextBatch,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.