diff --git a/packages/react-reconciler/src/ReactFiber.new.js b/packages/react-reconciler/src/ReactFiber.new.js
index 2401cd04cf3fd..fc9840538ca0e 100644
--- a/packages/react-reconciler/src/ReactFiber.new.js
+++ b/packages/react-reconciler/src/ReactFiber.new.js
@@ -24,6 +24,7 @@ import {
   enableStrictEffects,
   enableProfilerTimer,
   enableScopeAPI,
+  enableSyncDefaultUpdates,
 } from 'shared/ReactFeatureFlags';
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
 import {ConcurrentRoot} from './ReactRootTags';
@@ -442,8 +443,10 @@ export function createHostRootFiber(
         mode |= StrictLegacyMode;
       }
     }
-    if (concurrentUpdatesByDefaultOverride) {
-      mode |= ConcurrentUpdatesByDefaultMode;
+    if (enableSyncDefaultUpdates) {
+      if (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 7228b33710b66..db25edba8811a 100644
--- a/packages/react-reconciler/src/ReactFiber.old.js
+++ b/packages/react-reconciler/src/ReactFiber.old.js
@@ -24,6 +24,7 @@ import {
   enableStrictEffects,
   enableProfilerTimer,
   enableScopeAPI,
+  enableSyncDefaultUpdates,
 } from 'shared/ReactFeatureFlags';
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
 import {ConcurrentRoot} from './ReactRootTags';
@@ -442,8 +443,10 @@ export function createHostRootFiber(
         mode |= StrictLegacyMode;
       }
     }
-    if (concurrentUpdatesByDefaultOverride) {
-      mode |= ConcurrentUpdatesByDefaultMode;
+    if (enableSyncDefaultUpdates) {
+      if (concurrentUpdatesByDefaultOverride) {
+        mode |= ConcurrentUpdatesByDefaultMode;
+      }
     }
   } else {
     mode = NoMode;