diff --git a/fixtures/view-transition/src/components/Page.css b/fixtures/view-transition/src/components/Page.css
index e7d876681d..06100a53e8 100644
--- a/fixtures/view-transition/src/components/Page.css
+++ b/fixtures/view-transition/src/components/Page.css
@@ -20,4 +20,11 @@
   border: 0px;
   border-radius: 5px;
   padding: 10px;
+}
+
+.portal {
+  position: fixed;
+  top: 10px;
+  left: 360px;
+  border: 1px solid #ccc;
 }
\ No newline at end of file
diff --git a/fixtures/view-transition/src/components/Page.js b/fixtures/view-transition/src/components/Page.js
index d7b57c5110..9c158d5c77 100644
--- a/fixtures/view-transition/src/components/Page.js
+++ b/fixtures/view-transition/src/components/Page.js
@@ -6,7 +6,9 @@ import React, {
   useEffect,
   useState,
   useId,
+  startTransition,
 } from 'react';
+import {createPortal} from 'react-dom';
 
 import SwipeRecognizer from './SwipeRecognizer';
 
@@ -79,6 +81,23 @@ export default function Page({url, navigate}) {
     // });
   }, [show]);
 
+  const [showModal, setShowModal] = useState(false);
+  const portal = showModal ? (
+    createPortal(
+      <div className="portal">
+        Portal: {!show ? 'A' : 'B'}
+        <ViewTransition>
+          <div>{!show ? 'A' : 'B'}</div>
+        </ViewTransition>
+      </div>,
+      document.body
+    )
+  ) : (
+    <button onClick={() => startTransition(() => setShowModal(true))}>
+      Show Modal
+    </button>
+  );
+
   const exclamation = (
     <ViewTransition name="exclamation" onShare={onTransition}>
       <span>!</span>
@@ -153,6 +172,7 @@ export default function Page({url, navigate}) {
             <p>content</p>
             <p>out</p>
             <p>of</p>
+            {portal}
             <p>the</p>
             <p>viewport</p>
             {show ? <Component /> : null}
diff --git a/packages/react-reconciler/src/ReactFiberCommitWork.js b/packages/react-reconciler/src/ReactFiberCommitWork.js
index 5b27c8e494..700e12b329 100644
--- a/packages/react-reconciler/src/ReactFiberCommitWork.js
+++ b/packages/react-reconciler/src/ReactFiberCommitWork.js
@@ -283,6 +283,7 @@ export let shouldFireAfterActiveInstanceBlur: boolean = false;
 // Used during the commit phase to track whether a parent ViewTransition component
 // might have been affected by any mutations / relayouts below.
 let viewTransitionContextChanged: boolean = false;
+let rootViewTransitionAffected: boolean = false;
 
 export function commitBeforeMutationEffects(
   root: FiberRoot,
@@ -1750,6 +1751,8 @@ export function commitMutationEffects(
   inProgressLanes = committedLanes;
   inProgressRoot = root;
 
+  rootViewTransitionAffected = false;
+
   resetComponentEffectTimers();
 
   commitMutationEffectsOnFiber(finishedWork, root, committedLanes);
@@ -2068,6 +2071,7 @@ function commitMutationEffectsOnFiber(
       break;
     }
     case HostPortal: {
+      const prevMutationContext = pushMutationContext();
       if (supportsResources) {
         const previousHoistableRoot = currentHoistableRoot;
         currentHoistableRoot = getHoistableRoot(
@@ -2080,6 +2084,14 @@ function commitMutationEffectsOnFiber(
         recursivelyTraverseMutationEffects(root, finishedWork, lanes);
         commitReconciliationEffects(finishedWork, lanes);
       }
+      if (viewTransitionMutationContext) {
+        // A Portal doesn't necessarily exist within the context of this subtree.
+        // Ideally we would track which React ViewTransition component nests the container
+        // but that's costly. Instead, we treat each Portal as if it's a new React root.
+        // Therefore any leaked mutation means that the root should animate.
+        rootViewTransitionAffected = true;
+      }
+      popMutationContext(prevMutationContext);
 
       if (flags & Update) {
         if (supportsPersistence) {
@@ -2432,7 +2444,7 @@ function commitAfterMutationEffectsOnFiber(
       viewTransitionContextChanged = false;
       pushViewTransitionCancelableScope();
       recursivelyTraverseAfterMutationEffects(root, finishedWork, lanes);
-      if (!viewTransitionContextChanged) {
+      if (!viewTransitionContextChanged && !rootViewTransitionAffected) {
         // If we didn't leak any resizing out to the root, we don't have to transition
         // the root itself. This means that we can now safely cancel any cancellations
         // that bubbled all the way up.
@@ -2456,6 +2468,20 @@ function commitAfterMutationEffectsOnFiber(
       recursivelyTraverseAfterMutationEffects(root, finishedWork, lanes);
       break;
     }
+    case HostPortal: {
+      const prevContextChanged = viewTransitionContextChanged;
+      viewTransitionContextChanged = false;
+      recursivelyTraverseAfterMutationEffects(root, finishedWork, lanes);
+      if (viewTransitionContextChanged) {
+        // A Portal doesn't necessarily exist within the context of this subtree.
+        // Ideally we would track which React ViewTransition component nests the container
+        // but that's costly. Instead, we treat each Portal as if it's a new React root.
+        // Therefore any leaked resize of a child could affect the root so the root should animate.
+        rootViewTransitionAffected = true;
+      }
+      viewTransitionContextChanged = prevContextChanged;
+      break;
+    }
     case OffscreenComponent: {
       const isModernRoot =
         disableLegacyMode || (finishedWork.mode & ConcurrentMode) !== NoMode;