diff --git a/packages/react-dom/src/events/ReactDOMEventListener.js b/packages/react-dom/src/events/ReactDOMEventListener.js
index 3f00dcf305b37..f09d6ee864481 100644
--- a/packages/react-dom/src/events/ReactDOMEventListener.js
+++ b/packages/react-dom/src/events/ReactDOMEventListener.js
@@ -57,6 +57,9 @@ import {
   getCurrentUpdatePriority,
   setCurrentUpdatePriority,
 } from 'react-reconciler/src/ReactEventPriorities';
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
+const {ReactCurrentBatchConfig} = ReactSharedInternals;
 
 // TODO: can we stop exporting these?
 export let _enabled = true;
@@ -141,11 +144,14 @@ function dispatchContinuousEvent(
   nativeEvent,
 ) {
   const previousPriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
+  ReactCurrentBatchConfig.transition = 0;
   try {
     setCurrentUpdatePriority(ContinuousEventPriority);
     dispatchEvent(domEventName, eventSystemFlags, container, nativeEvent);
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
   }
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index cc41445830b40..b747392a61638 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -240,6 +240,7 @@ const ceil = Math.ceil;
 const {
   ReactCurrentDispatcher,
   ReactCurrentOwner,
+  ReactCurrentBatchConfig,
   IsSomeRendererActing,
 } = ReactSharedInternals;
 
@@ -1072,11 +1073,14 @@ export function flushDiscreteUpdates() {
 
 export function deferredUpdates<A>(fn: () => A): A {
   const previousPriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DefaultEventPriority);
     return fn();
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
   }
 }
 
@@ -1118,11 +1122,14 @@ export function discreteUpdates<A, B, C, D, R>(
   d: D,
 ): R {
   const previousPriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     return fn(a, b, c, d);
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
     if (executionContext === NoContext) {
       // Flush the immediate callbacks that were scheduled during this batch
       resetRenderTimer();
@@ -1151,8 +1158,10 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
 
+  const prevTransition = ReactCurrentBatchConfig.transition;
   const previousPriority = getCurrentUpdatePriority();
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     if (fn) {
       return fn(a);
@@ -1161,6 +1170,7 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     }
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
     executionContext = prevExecutionContext;
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
@@ -1182,12 +1192,15 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
 export function flushControlled(fn: () => mixed): void {
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
+  const prevTransition = ReactCurrentBatchConfig.transition;
   const previousPriority = getCurrentUpdatePriority();
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     fn();
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
 
     executionContext = prevExecutionContext;
     if (executionContext === NoContext) {
@@ -1681,10 +1694,13 @@ function commitRoot(root) {
   // TODO: This no longer makes any sense. We already wrap the mutation and
   // layout phases. Should be able to remove.
   const previousUpdateLanePriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     commitRootImpl(root, previousUpdateLanePriority);
   } finally {
+    ReactCurrentBatchConfig.transition = prevTransition;
     setCurrentUpdatePriority(previousUpdateLanePriority);
   }
 
@@ -1797,6 +1813,8 @@ function commitRootImpl(root, renderPriorityLevel) {
     NoFlags;
 
   if (subtreeHasEffects || rootHasEffect) {
+    const prevTransition = ReactCurrentBatchConfig.transition;
+    ReactCurrentBatchConfig.transition = 0;
     const previousPriority = getCurrentUpdatePriority();
     setCurrentUpdatePriority(DiscreteEventPriority);
 
@@ -1882,6 +1900,7 @@ function commitRootImpl(root, renderPriorityLevel) {
 
     // Reset the priority to the previous non-sync value.
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
   } else {
     // No effects.
     root.current = finishedWork;
@@ -2022,12 +2041,15 @@ export function flushPassiveEffects(): boolean {
   if (rootWithPendingPassiveEffects !== null) {
     const renderPriority = lanesToEventPriority(pendingPassiveEffectsLanes);
     const priority = lowerEventPriority(DefaultEventPriority, renderPriority);
+    const prevTransition = ReactCurrentBatchConfig.transition;
     const previousPriority = getCurrentUpdatePriority();
     try {
+      ReactCurrentBatchConfig.transition = 0;
       setCurrentUpdatePriority(priority);
       return flushPassiveEffectsImpl();
     } finally {
       setCurrentUpdatePriority(previousPriority);
+      ReactCurrentBatchConfig.transition = prevTransition;
     }
   }
   return false;
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 4420e51aa1e2c..35250f1d6d526 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -240,6 +240,7 @@ const ceil = Math.ceil;
 const {
   ReactCurrentDispatcher,
   ReactCurrentOwner,
+  ReactCurrentBatchConfig,
   IsSomeRendererActing,
 } = ReactSharedInternals;
 
@@ -1072,11 +1073,14 @@ export function flushDiscreteUpdates() {
 
 export function deferredUpdates<A>(fn: () => A): A {
   const previousPriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DefaultEventPriority);
     return fn();
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
   }
 }
 
@@ -1118,11 +1122,14 @@ export function discreteUpdates<A, B, C, D, R>(
   d: D,
 ): R {
   const previousPriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     return fn(a, b, c, d);
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
     if (executionContext === NoContext) {
       // Flush the immediate callbacks that were scheduled during this batch
       resetRenderTimer();
@@ -1151,8 +1158,10 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
 
+  const prevTransition = ReactCurrentBatchConfig.transition;
   const previousPriority = getCurrentUpdatePriority();
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     if (fn) {
       return fn(a);
@@ -1161,6 +1170,7 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     }
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
     executionContext = prevExecutionContext;
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
@@ -1182,12 +1192,15 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
 export function flushControlled(fn: () => mixed): void {
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
+  const prevTransition = ReactCurrentBatchConfig.transition;
   const previousPriority = getCurrentUpdatePriority();
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     fn();
   } finally {
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
 
     executionContext = prevExecutionContext;
     if (executionContext === NoContext) {
@@ -1681,10 +1694,13 @@ function commitRoot(root) {
   // TODO: This no longer makes any sense. We already wrap the mutation and
   // layout phases. Should be able to remove.
   const previousUpdateLanePriority = getCurrentUpdatePriority();
+  const prevTransition = ReactCurrentBatchConfig.transition;
   try {
+    ReactCurrentBatchConfig.transition = 0;
     setCurrentUpdatePriority(DiscreteEventPriority);
     commitRootImpl(root, previousUpdateLanePriority);
   } finally {
+    ReactCurrentBatchConfig.transition = prevTransition;
     setCurrentUpdatePriority(previousUpdateLanePriority);
   }
 
@@ -1797,6 +1813,8 @@ function commitRootImpl(root, renderPriorityLevel) {
     NoFlags;
 
   if (subtreeHasEffects || rootHasEffect) {
+    const prevTransition = ReactCurrentBatchConfig.transition;
+    ReactCurrentBatchConfig.transition = 0;
     const previousPriority = getCurrentUpdatePriority();
     setCurrentUpdatePriority(DiscreteEventPriority);
 
@@ -1882,6 +1900,7 @@ function commitRootImpl(root, renderPriorityLevel) {
 
     // Reset the priority to the previous non-sync value.
     setCurrentUpdatePriority(previousPriority);
+    ReactCurrentBatchConfig.transition = prevTransition;
   } else {
     // No effects.
     root.current = finishedWork;
@@ -2022,12 +2041,15 @@ export function flushPassiveEffects(): boolean {
   if (rootWithPendingPassiveEffects !== null) {
     const renderPriority = lanesToEventPriority(pendingPassiveEffectsLanes);
     const priority = lowerEventPriority(DefaultEventPriority, renderPriority);
+    const prevTransition = ReactCurrentBatchConfig.transition;
     const previousPriority = getCurrentUpdatePriority();
     try {
+      ReactCurrentBatchConfig.transition = 0;
       setCurrentUpdatePriority(priority);
       return flushPassiveEffectsImpl();
     } finally {
       setCurrentUpdatePriority(previousPriority);
+      ReactCurrentBatchConfig.transition = prevTransition;
     }
   }
   return false;
diff --git a/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js b/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js
index 3c96f5ee65914..bc7499a3d43a7 100644
--- a/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js
+++ b/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js
@@ -3,6 +3,7 @@ let ReactNoop;
 let Scheduler;
 let useState;
 let useEffect;
+let startTransition;
 
 describe('ReactFlushSync', () => {
   beforeEach(() => {
@@ -13,6 +14,7 @@ describe('ReactFlushSync', () => {
     Scheduler = require('scheduler');
     useState = React.useState;
     useEffect = React.useEffect;
+    startTransition = React.unstable_startTransition;
   });
 
   function Text({text}) {
@@ -54,4 +56,45 @@ describe('ReactFlushSync', () => {
     });
     expect(root).toMatchRenderedOutput('1, 1');
   });
+
+  // @gate experimental
+  test('nested with startTransition', async () => {
+    let setSyncState;
+    let setState;
+    function App() {
+      const [syncState, _setSyncState] = useState(0);
+      const [state, _setState] = useState(0);
+      setSyncState = _setSyncState;
+      setState = _setState;
+      return <Text text={`${syncState}, ${state}`} />;
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+    });
+    expect(Scheduler).toHaveYielded(['0, 0']);
+    expect(root).toMatchRenderedOutput('0, 0');
+
+    await ReactNoop.act(async () => {
+      ReactNoop.flushSync(() => {
+        startTransition(() => {
+          // This should be async even though flushSync is on the stack, because
+          // startTransition is closer.
+          setState(1);
+          ReactNoop.flushSync(() => {
+            // This should be async even though startTransition is on the stack,
+            // because flushSync is closer.
+            setSyncState(1);
+          });
+        });
+      });
+      // Only the sync update should have flushed
+      expect(Scheduler).toHaveYielded(['1, 0']);
+      expect(root).toMatchRenderedOutput('1, 0');
+    });
+    // Now the async update has flushed, too.
+    expect(Scheduler).toHaveYielded(['1, 1']);
+    expect(root).toMatchRenderedOutput('1, 1');
+  });
 });