diff --git a/change/@fluentui-react-portal-ea64a7c1-d032-4c04-a5b8-083f1685c723.json b/change/@fluentui-react-portal-ea64a7c1-d032-4c04-a5b8-083f1685c723.json
new file mode 100644
index 00000000000000..0c1107003b8a57
--- /dev/null
+++ b/change/@fluentui-react-portal-ea64a7c1-d032-4c04-a5b8-083f1685c723.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "refactor: remove Griffel dependency from usePortalMountNode",
+  "packageName": "@fluentui/react-portal",
+  "email": "dmytrokirpa@microsoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNode.ts b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNode.ts
index 60f46090a85790..adb6cf794765b8 100644
--- a/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNode.ts
+++ b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNode.ts
@@ -6,10 +6,9 @@ import {
   useFluent_unstable as useFluent,
   usePortalMountNode as usePortalMountNodeContext,
 } from '@fluentui/react-shared-contexts';
-import { mergeClasses } from '@griffel/react';
 import { useFocusVisible } from '@fluentui/react-tabster';
 
-import { usePortalMountNodeStylesStyles } from './usePortalMountNodeStyles.styles';
+import { usePortalMountNodeStyles } from './usePortalMountNodeStyles';
 
 const useInsertionEffect = (React as never)['useInsertion' + 'Effect'] as typeof React.useLayoutEffect | undefined;
 
@@ -198,16 +197,14 @@ const useModernElementFactory: UseElementFactory = options => {
       return;
     }
 
-    const classesToApply = className.split(' ').filter(Boolean);
-
-    elementProxy.classList.add(...classesToApply);
+    elementProxy.setAttribute('class', className);
     elementProxy.setAttribute('dir', dir);
     elementProxy.setAttribute('data-portal-node', 'true');
 
     focusVisibleRef.current = elementProxy;
 
     return () => {
-      elementProxy.classList.remove(...classesToApply);
+      elementProxy.removeAttribute('class');
       elementProxy.removeAttribute('dir');
     };
   }, [className, dir, elementProxy, focusVisibleRef]);
@@ -243,7 +240,6 @@ export const usePortalMountNode = (options: UsePortalMountNodeOptions): HTMLElem
 
   // eslint-disable-next-line @typescript-eslint/no-deprecated
   const focusVisibleRef = useFocusVisible<HTMLDivElement>() as React.MutableRefObject<HTMLElement | null>;
-  const classes = usePortalMountNodeStylesStyles();
   const themeClassName = useThemeClassName();
 
   const factoryOptions: UseElementFactoryOptions = {
@@ -251,9 +247,11 @@ export const usePortalMountNode = (options: UsePortalMountNodeOptions): HTMLElem
     disabled: options.disabled,
     focusVisibleRef,
 
-    className: mergeClasses(themeClassName, classes.root, options.className),
+    className: [themeClassName, options.className].filter(Boolean).join(' '),
     targetNode: mountNode ?? targetDocument?.body,
   };
 
+  usePortalMountNodeStyles(options.disabled);
+
   return useElementFactory(factoryOptions);
 };
diff --git a/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.styles.ts b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.styles.ts
deleted file mode 100644
index ed71de80786792..00000000000000
--- a/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.styles.ts
+++ /dev/null
@@ -1,18 +0,0 @@
-'use client';
-
-import { makeStyles } from '@griffel/react';
-
-export const usePortalMountNodeStylesStyles = makeStyles({
-  root: {
-    // Creates new stacking context to prevent z-index issues
-    // https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout/Understanding_z-index/Stacking_context
-    //
-    // Also keeps a portal on top of a page to prevent scrollbars from appearing
-    position: 'absolute',
-    top: 0,
-    left: 0,
-    right: 0,
-
-    zIndex: 1000000,
-  },
-});
diff --git a/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.test.tsx b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.test.tsx
new file mode 100644
index 00000000000000..af3093466b5189
--- /dev/null
+++ b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.test.tsx
@@ -0,0 +1,97 @@
+import { renderHook } from '@testing-library/react-hooks';
+import { Provider_unstable as Provider } from '@fluentui/react-shared-contexts';
+import * as React from 'react';
+
+import { usePortalMountNodeStyles, PORTAL_STYLE_ELEMENT_ID, setPortalRefCount } from './usePortalMountNodeStyles';
+
+function queryStyleElement(): HTMLStyleElement | null {
+  return document.head.querySelector(`#${PORTAL_STYLE_ELEMENT_ID}`);
+}
+
+function createWrapper(targetDocument: Document | undefined) {
+  return (props: { children?: React.ReactNode }) => (
+    <Provider value={{ dir: 'ltr', targetDocument }}>{props.children}</Provider>
+  );
+}
+
+describe('usePortalMountNodeStyles', () => {
+  afterEach(() => {
+    // Clean up any leftover style elements and reset the ref count
+    queryStyleElement()?.remove();
+    setPortalRefCount(document, 0);
+  });
+
+  it('injects a <style> element into document.head when enabled', () => {
+    expect(queryStyleElement()).toBeNull();
+
+    renderHook(() => usePortalMountNodeStyles(false));
+
+    const style = queryStyleElement();
+    expect(style).not.toBeNull();
+    expect(style!.parentElement).toBe(document.head);
+  });
+
+  it('does not inject a <style> element when disabled', () => {
+    renderHook(() => usePortalMountNodeStyles(true));
+
+    expect(queryStyleElement()).toBeNull();
+  });
+
+  it('does not inject a <style> element when targetDocument is undefined', () => {
+    renderHook(() => usePortalMountNodeStyles(false), {
+      wrapper: createWrapper(undefined),
+    });
+
+    expect(queryStyleElement()).toBeNull();
+  });
+
+  it('removes the <style> element on unmount', () => {
+    const { unmount } = renderHook(() => usePortalMountNodeStyles(false));
+
+    expect(queryStyleElement()).not.toBeNull();
+
+    unmount();
+
+    expect(queryStyleElement()).toBeNull();
+  });
+
+  it('shares a single <style> element across multiple consumers', () => {
+    const hook1 = renderHook(() => usePortalMountNodeStyles(false));
+    const hook2 = renderHook(() => usePortalMountNodeStyles(false));
+
+    const allStyles = document.head.querySelectorAll(`#${PORTAL_STYLE_ELEMENT_ID}`);
+    expect(allStyles.length).toBe(1);
+
+    // Unmounting one consumer keeps the style
+    hook1.unmount();
+    expect(queryStyleElement()).not.toBeNull();
+
+    // Unmounting the last consumer removes it
+    hook2.unmount();
+    expect(queryStyleElement()).toBeNull();
+  });
+
+  it('injects the style rule via insertRule', () => {
+    renderHook(() => usePortalMountNodeStyles(false));
+
+    const style = queryStyleElement();
+    expect(style).not.toBeNull();
+    expect(style!.sheet).not.toBeNull();
+    expect(style!.sheet!.cssRules.length).toBe(1);
+    expect(style!.sheet!.cssRules[0].cssText).toContain('[data-portal-node]');
+  });
+
+  it('prepends the <style> element as the first child of head', () => {
+    // Add an existing element to head
+    const existing = document.createElement('link');
+    document.head.appendChild(existing);
+
+    renderHook(() => usePortalMountNodeStyles(false));
+
+    const style = queryStyleElement();
+    expect(style).not.toBeNull();
+    expect(document.head.firstElementChild).toBe(style);
+
+    existing.remove();
+  });
+});
diff --git a/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.ts b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.ts
new file mode 100644
index 00000000000000..72564902924765
--- /dev/null
+++ b/packages/react-components/react-portal/library/src/components/Portal/usePortalMountNodeStyles.ts
@@ -0,0 +1,83 @@
+'use client';
+
+import * as React from 'react';
+import { useFluent_unstable as useFluent } from '@fluentui/react-shared-contexts';
+import { useIsomorphicLayoutEffect } from '@fluentui/react-utilities';
+
+// String concatenation is used to prevent bundlers to complain with older versions of React
+const useInsertionEffect = (React as never)['useInsertion' + 'Effect']
+  ? (React as never)['useInsertion' + 'Effect']
+  : useIsomorphicLayoutEffect;
+
+// Symbol used as a "private" property key on Document to store the active portal reference count.
+// Symbol.for() registers in the global Symbol registry so the same key is shared across bundles
+// (e.g. when multiple copies of this module are loaded in the same page).
+// Storing state directly on the document avoids any WeakMap cross-reference issues and is safe
+// across multiple documents (e.g. iframes) because each document object carries its own counter.
+const PORTAL_STYLE_REF_COUNT = Symbol.for('fui-portal-style-ref-count');
+
+type DocumentWithPortalCounter = Document & { [PORTAL_STYLE_REF_COUNT]?: number };
+
+// Creates new stacking context to prevent z-index issues
+// https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout/Understanding_z-index/Stacking_context
+//
+// Also keeps a portal on top of a page to prevent scrollbars from appearing
+const PORTAL_MOUNT_NODE_STYLE_RULE = `[data-portal-node]{position:absolute;top:0;left:0;right:0;z-index:1000000}`;
+
+// ID used to identify the injected portal mount node <style> element in a document.
+// Only one such element exists per document, so an id is appropriate.
+export const PORTAL_STYLE_ELEMENT_ID = 'fui-portal-styles';
+
+export function getPortalRefCount(targetDocument: Document): number {
+  return (targetDocument as DocumentWithPortalCounter)[PORTAL_STYLE_REF_COUNT] ?? 0;
+}
+
+export function setPortalRefCount(targetDocument: Document, count: number): void {
+  (targetDocument as DocumentWithPortalCounter)[PORTAL_STYLE_REF_COUNT] = count;
+}
+
+function injectPortalMountNodeStyles(targetDocument: Document): void {
+  const currentCount = getPortalRefCount(targetDocument);
+  if (currentCount > 0) {
+    setPortalRefCount(targetDocument, currentCount + 1);
+    return;
+  }
+  const style = targetDocument.createElement('style');
+  style.id = PORTAL_STYLE_ELEMENT_ID;
+  // Prepend so that consumer class names (applied later in document order) can override these
+  // defaults via CSS source order at equal specificity — the same cascade behaviour as before.
+  // Both prepend and append trigger one style recalculation; position in <head> does not change
+  // the number of recalcs.
+  targetDocument.head.prepend(style);
+  // sheet is available after the element is inserted into the document
+  style.sheet?.insertRule(PORTAL_MOUNT_NODE_STYLE_RULE);
+  setPortalRefCount(targetDocument, 1);
+}
+
+function ejectPortalMountNodeStyles(targetDocument: Document): void {
+  const currentCount = getPortalRefCount(targetDocument);
+  if (currentCount === 0) {
+    return;
+  }
+  const newCount = currentCount - 1;
+  if (newCount === 0) {
+    targetDocument.head.querySelector(`#${PORTAL_STYLE_ELEMENT_ID}`)?.remove();
+  }
+  setPortalRefCount(targetDocument, newCount);
+}
+
+/**
+ * Injects a shared <style> element for portal mount node styling into the target document,
+ * and removes it when the last consumer unmounts (reference counted via a Symbol property on `document`).
+ */
+export function usePortalMountNodeStyles(disabled: boolean | undefined): void {
+  const { targetDocument } = useFluent();
+
+  useInsertionEffect!(() => {
+    if (disabled || !targetDocument) {
+      return;
+    }
+    injectPortalMountNodeStyles(targetDocument);
+    return () => ejectPortalMountNodeStyles(targetDocument);
+  }, [disabled, targetDocument]);
+}