diff --git a/packages/react-reconciler/src/ReactFiber.new.js b/packages/react-reconciler/src/ReactFiber.new.js
index 2ea25195a7c02..a18514c50c0f2 100644
--- a/packages/react-reconciler/src/ReactFiber.new.js
+++ b/packages/react-reconciler/src/ReactFiber.new.js
@@ -492,7 +492,11 @@ export function createFiberFromTypeAndProps(
         break;
       case REACT_STRICT_MODE_TYPE:
         fiberTag = Mode;
-        mode |= StrictLegacyMode | StrictEffectsMode;
+        mode |= StrictLegacyMode;
+        if (enableStrictEffects && (mode & ConcurrentMode) !== NoMode) {
+          // Strict effects should never run on legacy roots
+          mode |= StrictEffectsMode;
+        }
         break;
       case REACT_PROFILER_TYPE:
         return createFiberFromProfiler(pendingProps, mode, lanes, key);
diff --git a/packages/react-reconciler/src/ReactFiber.old.js b/packages/react-reconciler/src/ReactFiber.old.js
index 1e30bb9c7a3a3..4fbbb5a00efca 100644
--- a/packages/react-reconciler/src/ReactFiber.old.js
+++ b/packages/react-reconciler/src/ReactFiber.old.js
@@ -492,7 +492,11 @@ export function createFiberFromTypeAndProps(
         break;
       case REACT_STRICT_MODE_TYPE:
         fiberTag = Mode;
-        mode |= StrictLegacyMode | StrictEffectsMode;
+        mode |= StrictLegacyMode;
+        if (enableStrictEffects && (mode & ConcurrentMode) !== NoMode) {
+          // Strict effects should never run on legacy roots
+          mode |= StrictEffectsMode;
+        }
         break;
       case REACT_PROFILER_TYPE:
         return createFiberFromProfiler(pendingProps, mode, lanes, key);
diff --git a/packages/react-reconciler/src/ReactFiberClassComponent.new.js b/packages/react-reconciler/src/ReactFiberClassComponent.new.js
index 44b9db7aacb27..c464b6c3bf411 100644
--- a/packages/react-reconciler/src/ReactFiberClassComponent.new.js
+++ b/packages/react-reconciler/src/ReactFiberClassComponent.new.js
@@ -923,7 +923,6 @@ function mountClassInstance(
       enableStrictEffects &&
       (workInProgress.mode & StrictEffectsMode) !== NoMode
     ) {
-      // Never double-invoke effects for legacy roots.
       fiberFlags |= MountLayoutDev;
     }
     workInProgress.flags |= fiberFlags;
@@ -1005,7 +1004,6 @@ function resumeMountClassInstance(
         enableStrictEffects &&
         (workInProgress.mode & StrictEffectsMode) !== NoMode
       ) {
-        // Never double-invoke effects for legacy roots.
         fiberFlags |= MountLayoutDev;
       }
       workInProgress.flags |= fiberFlags;
@@ -1060,7 +1058,6 @@ function resumeMountClassInstance(
         enableStrictEffects &&
         (workInProgress.mode & StrictEffectsMode) !== NoMode
       ) {
-        // Never double-invoke effects for legacy roots.
         fiberFlags |= MountLayoutDev;
       }
       workInProgress.flags |= fiberFlags;
@@ -1078,7 +1075,6 @@ function resumeMountClassInstance(
         enableStrictEffects &&
         (workInProgress.mode & StrictEffectsMode) !== NoMode
       ) {
-        // Never double-invoke effects for legacy roots.
         fiberFlags |= MountLayoutDev;
       }
       workInProgress.flags |= fiberFlags;
diff --git a/packages/react-reconciler/src/ReactFiberClassComponent.old.js b/packages/react-reconciler/src/ReactFiberClassComponent.old.js
index ab2bb43796781..adb616c6262b9 100644
--- a/packages/react-reconciler/src/ReactFiberClassComponent.old.js
+++ b/packages/react-reconciler/src/ReactFiberClassComponent.old.js
@@ -923,7 +923,6 @@ function mountClassInstance(
       enableStrictEffects &&
       (workInProgress.mode & StrictEffectsMode) !== NoMode
     ) {
-      // Never double-invoke effects for legacy roots.
       fiberFlags |= MountLayoutDev;
     }
     workInProgress.flags |= fiberFlags;
@@ -1005,7 +1004,6 @@ function resumeMountClassInstance(
         enableStrictEffects &&
         (workInProgress.mode & StrictEffectsMode) !== NoMode
       ) {
-        // Never double-invoke effects for legacy roots.
         fiberFlags |= MountLayoutDev;
       }
       workInProgress.flags |= fiberFlags;
@@ -1060,7 +1058,6 @@ function resumeMountClassInstance(
         enableStrictEffects &&
         (workInProgress.mode & StrictEffectsMode) !== NoMode
       ) {
-        // Never double-invoke effects for legacy roots.
         fiberFlags |= MountLayoutDev;
       }
       workInProgress.flags |= fiberFlags;
@@ -1078,7 +1075,6 @@ function resumeMountClassInstance(
         enableStrictEffects &&
         (workInProgress.mode & StrictEffectsMode) !== NoMode
       ) {
-        // Never double-invoke effects for legacy roots.
         fiberFlags |= MountLayoutDev;
       }
       workInProgress.flags |= fiberFlags;