diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index aa36d6da831e7..b6f697edfee38 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -1082,6 +1082,15 @@ export function flushSyncWithoutWarningIfAlreadyRendering<A, R>(
   fn: A => R,
   a: A,
 ): R {
+  // In legacy mode, we flush pending passive effects at the beginning of the
+  // next event, not at the end of the previous one.
+  if (
+    rootWithPendingPassiveEffects !== null &&
+    rootWithPendingPassiveEffects.tag === LegacyRoot
+  ) {
+    flushPassiveEffects();
+  }
+
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;
 
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 2fd46c7cda7ca..22c8395cb0988 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -1082,6 +1082,15 @@ export function flushSyncWithoutWarningIfAlreadyRendering<A, R>(
   fn: A => R,
   a: A,
 ): R {
+  // In legacy mode, we flush pending passive effects at the beginning of the
+  // next event, not at the end of the previous one.
+  if (
+    rootWithPendingPassiveEffects !== null &&
+    rootWithPendingPassiveEffects.tag === LegacyRoot
+  ) {
+    flushPassiveEffects();
+  }
+
   const prevExecutionContext = executionContext;
   executionContext |= BatchedContext;