diff --git a/packages/react-reconciler/src/ReactFiberReconciler.js b/packages/react-reconciler/src/ReactFiberReconciler.js
index 65eebf04e48..d58371d97ba 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.js
@@ -470,6 +470,21 @@ export {
   flushPendingEffects as flushPassiveEffects,
 };
 
+// Deprecated: Add deprecation warning for removed flushSync API
+let didWarnAboutFlushSync = false;
+export const flushSync = __DEV__
+  ? function flushSync() {
+      if (!didWarnAboutFlushSync) {
+        didWarnAboutFlushSync = true;
+        console.error(
+          'The `flushSync` export from react-reconciler has been removed. ' +
+            'Use `updateContainerSync()` followed by `flushSyncWork()` instead. ' +
+            'See https://github.com/facebook/react/pull/28500 for more details.',
+        );
+      }
+    }
+  : undefined;
+
 export function getPublicRootInstance(
   container: OpaqueRoot,
 ): component(...props: any) | PublicInstance | null {
diff --git a/packages/react-reconciler/src/__tests__/ReactReconcilerFlushSyncDeprecation-test.js b/packages/react-reconciler/src/__tests__/ReactReconcilerFlushSyncDeprecation-test.js
new file mode 100644
index 00000000000..7f63f013a0e
--- /dev/null
+++ b/packages/react-reconciler/src/__tests__/ReactReconcilerFlushSyncDeprecation-test.js
@@ -0,0 +1,30 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+let Reconciler;
+let assertConsoleErrorDev;
+
+describe('ReactReconciler flushSync deprecation', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    Reconciler = require('react-reconciler');
+    const InternalTestUtils = require('internal-test-utils');
+    assertConsoleErrorDev = InternalTestUtils.assertConsoleErrorDev;
+  });
+
+  // @gate __DEV__
+  it('warns when accessing the deprecated flushSync export', () => {
+    const flushSync = Reconciler.flushSync;
+    expect(typeof flushSync).toBe('function');
+    flushSync();
+    assertConsoleErrorDev([
+      'The `flushSync` export from react-reconciler has been removed. ' +
+        'Use `updateContainerSync()` followed by `flushSyncWork()` instead. ' +
+        'See https://github.com/facebook/react/pull/28500 for more details.',
+    ]);
+  });
+});