diff --git a/.changeset/portal-css-containment.md b/.changeset/portal-css-containment.md
new file mode 100644
index 00000000000..c11b9f7be0c
--- /dev/null
+++ b/.changeset/portal-css-containment.md
@@ -0,0 +1,5 @@
+---
+"@primer/react": minor
+---
+
+Added `primer_react_css_contain_portal` feature flag that applies CSS containment (`contain: layout style`) to Portal elements when enabled. This can improve rendering performance by isolating layout and style calculations within portals without clipping overflow content.
diff --git a/packages/react/src/Portal/Portal.test.tsx b/packages/react/src/Portal/Portal.test.tsx
index 7b4975b4616..05635e12396 100644
--- a/packages/react/src/Portal/Portal.test.tsx
+++ b/packages/react/src/Portal/Portal.test.tsx
@@ -1,9 +1,10 @@
-import {describe, expect, it} from 'vitest'
-import Portal, {registerPortalRoot, PortalContext} from '../Portal/index'
+import {describe, expect, it, vi, beforeEach} from 'vitest'
+import Portal, {registerPortalRoot, PortalContext, resetCSSContainmentTracking} from '../Portal/index'
 
 import {render} from '@testing-library/react'
 import BaseStyles from '../BaseStyles'
 import React from 'react'
+import {FeatureFlags} from '../FeatureFlags'
 
 describe('Portal', () => {
   it('renders a default portal into document.body (no BaseStyles present)', () => {
@@ -183,4 +184,100 @@ describe('Portal', () => {
     document.body.removeChild(contextPortalRoot)
     document.body.removeChild(propPortalRoot)
   })
+
+  describe('CSS containment feature flag', () => {
+    beforeEach(() => {
+      // Reset containment tracking between tests
+      resetCSSContainmentTracking()
+    })
+
+    it('does not apply CSS containment by default', () => {
+      const {baseElement} = render(<Portal>test-content</Portal>)
+      const generatedRoot = baseElement.querySelector('#__primerPortalRoot__') as HTMLElement
+
+      expect(generatedRoot).toBeInstanceOf(HTMLElement)
+      expect(generatedRoot.style.contain).toBe('')
+
+      baseElement.innerHTML = ''
+    })
+
+    it('applies CSS containment when primer_react_css_contain_portal flag is enabled', () => {
+      const toRender = (
+        <FeatureFlags flags={{primer_react_css_contain_portal: true}}>
+          <Portal>contained-content</Portal>
+        </FeatureFlags>
+      )
+
+      const {baseElement} = render(toRender)
+      const generatedRoot = baseElement.querySelector('#__primerPortalRoot__') as HTMLElement
+
+      expect(generatedRoot).toBeInstanceOf(HTMLElement)
+      expect(generatedRoot.style.contain).toBe('layout style')
+
+      baseElement.innerHTML = ''
+    })
+
+    it('does not apply CSS containment when flag is explicitly disabled', () => {
+      const toRender = (
+        <FeatureFlags flags={{primer_react_css_contain_portal: false}}>
+          <Portal>uncontained-content</Portal>
+        </FeatureFlags>
+      )
+
+      const {baseElement} = render(toRender)
+      const generatedRoot = baseElement.querySelector('#__primerPortalRoot__') as HTMLElement
+
+      expect(generatedRoot).toBeInstanceOf(HTMLElement)
+      expect(generatedRoot.style.contain).toBe('')
+
+      baseElement.innerHTML = ''
+    })
+
+    it('applies CSS containment to pre-existing portal root', () => {
+      // Create a pre-existing portal root declaratively
+      const existingRoot = document.createElement('div')
+      existingRoot.id = '__primerPortalRoot__'
+      document.body.appendChild(existingRoot)
+
+      const toRender = (
+        <FeatureFlags flags={{primer_react_css_contain_portal: true}}>
+          <Portal>content-in-existing-root</Portal>
+        </FeatureFlags>
+      )
+
+      const {baseElement} = render(toRender)
+      const generatedRoot = baseElement.querySelector('#__primerPortalRoot__') as HTMLElement
+
+      expect(generatedRoot).toBeInstanceOf(HTMLElement)
+      expect(generatedRoot.style.contain).toBe('layout style')
+
+      baseElement.innerHTML = ''
+      document.body.removeChild(existingRoot)
+    })
+
+    it('warns when overriding existing contain value', () => {
+      const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {})
+
+      // Create a pre-existing portal root with a different contain value
+      const existingRoot = document.createElement('div')
+      existingRoot.id = '__primerPortalRoot__'
+      existingRoot.style.contain = 'size'
+      document.body.appendChild(existingRoot)
+
+      const toRender = (
+        <FeatureFlags flags={{primer_react_css_contain_portal: true}}>
+          <Portal>content-with-override</Portal>
+        </FeatureFlags>
+      )
+
+      render(toRender)
+
+      expect(warnSpy).toHaveBeenCalledWith(
+        'Portal root already has contain: "size". Overriding with "layout style" due to primer_react_css_contain_portal flag.',
+      )
+
+      warnSpy.mockRestore()
+      document.body.removeChild(existingRoot)
+    })
+  })
 })
diff --git a/packages/react/src/Portal/Portal.tsx b/packages/react/src/Portal/Portal.tsx
index 68797db5a15..c142ad21794 100644
--- a/packages/react/src/Portal/Portal.tsx
+++ b/packages/react/src/Portal/Portal.tsx
@@ -1,12 +1,25 @@
 import React, {useContext} from 'react'
 import {createPortal} from 'react-dom'
 import useLayoutEffect from '../utils/useIsomorphicLayoutEffect'
+import {useFeatureFlag} from '../FeatureFlags'
 
 const PRIMER_PORTAL_ROOT_ID = '__primerPortalRoot__'
 const DEFAULT_PORTAL_CONTAINER_NAME = '__default__'
 
 const portalRootRegistry: Partial<Record<string, Element>> = {}
 
+// Track which portal roots have had CSS containment applied (auto-cleans when element is GC'd)
+const cssContainmentApplied = new WeakMap<Element, boolean>()
+
+// Reset containment tracking (exported for testing)
+export function resetCSSContainmentTracking(): void {
+  // WeakMap doesn't have a clear method, but we can reset by deleting the current portal root entry
+  const portalRoot = portalRootRegistry[DEFAULT_PORTAL_CONTAINER_NAME]
+  if (portalRoot) {
+    cssContainmentApplied.delete(portalRoot)
+  }
+}
+
 /**
  * Register a container to serve as a portal root.
  * @param root The element that will be the root for portals created in this container
@@ -20,7 +33,7 @@ export function registerPortalRoot(root: Element, name = DEFAULT_PORTAL_CONTAINE
 // Ensures that a default portal root exists and is registered. If a DOM element exists
 // with id __primerPortalRoot__, allow that element to serve as the default portal root.
 // Otherwise, create that element and attach it to the end of document.body.
-function ensureDefaultPortal() {
+function ensureDefaultPortal(enableCSSContainment = false) {
   const existingDefaultPortalContainer = portalRootRegistry[DEFAULT_PORTAL_CONTAINER_NAME]
   if (!existingDefaultPortalContainer || !document.body.contains(existingDefaultPortalContainer)) {
     let defaultPortalContainer = document.getElementById(PRIMER_PORTAL_ROOT_ID)
@@ -41,6 +54,22 @@ function ensureDefaultPortal() {
 
     registerPortalRoot(defaultPortalContainer)
   }
+
+  // Apply CSS containment to the portal root if enabled (only once per root)
+  if (enableCSSContainment) {
+    const portalRoot = portalRootRegistry[DEFAULT_PORTAL_CONTAINER_NAME]
+    if (portalRoot instanceof HTMLElement && !cssContainmentApplied.has(portalRoot)) {
+      const existingContain = portalRoot.style.contain
+      if (existingContain && existingContain !== 'layout style') {
+        // eslint-disable-next-line no-console
+        console.warn(
+          `Portal root already has contain: "${existingContain}". Overriding with "layout style" due to primer_react_css_contain_portal flag.`,
+        )
+      }
+      portalRoot.style.contain = 'layout style'
+      cssContainmentApplied.set(portalRoot, true)
+    }
+  }
 }
 
 /**
@@ -75,6 +104,7 @@ export const Portal: React.FC<React.PropsWithChildren<PortalProps>> = ({
   containerName: _containerName,
 }) => {
   const {portalContainerName} = useContext(PortalContext)
+  const enableCSSContainment = useFeatureFlag('primer_react_css_contain_portal')
   const elementRef = React.useRef<HTMLDivElement | null>(null)
   if (!elementRef.current) {
     const div = document.createElement('div')
@@ -92,7 +122,7 @@ export const Portal: React.FC<React.PropsWithChildren<PortalProps>> = ({
     let containerName = _containerName ?? portalContainerName
     if (containerName === undefined) {
       containerName = DEFAULT_PORTAL_CONTAINER_NAME
-      ensureDefaultPortal()
+      ensureDefaultPortal(enableCSSContainment)
     }
     const parentElement = portalRootRegistry[containerName]
 
diff --git a/packages/react/src/Portal/index.ts b/packages/react/src/Portal/index.ts
index 375dce81355..493f74ac3b5 100644
--- a/packages/react/src/Portal/index.ts
+++ b/packages/react/src/Portal/index.ts
@@ -1,7 +1,7 @@
 import type {PortalProps} from './Portal'
-import {Portal, registerPortalRoot, PortalContext} from './Portal'
+import {Portal, registerPortalRoot, PortalContext, resetCSSContainmentTracking} from './Portal'
 
 export default Portal
-export {registerPortalRoot}
+export {registerPortalRoot, resetCSSContainmentTracking}
 export type {PortalProps}
 export {PortalContext}