diff --git a/change/@fluentui-react-message-bar-92344a29-0bc4-4716-a66c-be75a48e33b1.json b/change/@fluentui-react-message-bar-92344a29-0bc4-4716-a66c-be75a48e33b1.json
new file mode 100644
index 00000000000000..d111f8d0ba8ed0
--- /dev/null
+++ b/change/@fluentui-react-message-bar-92344a29-0bc4-4716-a66c-be75a48e33b1.json
@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "fix(MessageBar): v9 MessageBar Resize Flicker Fix",
+  "packageName": "@fluentui/react-message-bar",
+  "email": "jiangemma@microsoft.com",
+  "dependentChangeType": "patch"
+}
diff --git a/packages/react-components/react-message-bar/library/src/components/MessageBar/MessageBar.test.tsx b/packages/react-components/react-message-bar/library/src/components/MessageBar/MessageBar.test.tsx
index 1fac6cac4f3815..389a4cdc6de0ff 100644
--- a/packages/react-components/react-message-bar/library/src/components/MessageBar/MessageBar.test.tsx
+++ b/packages/react-components/react-message-bar/library/src/components/MessageBar/MessageBar.test.tsx
@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { render } from '@testing-library/react';
+import { render, act } from '@testing-library/react';
 import { isConformant } from '../../testing/isConformant';
 import { MessageBar } from './MessageBar';
 import { AnnounceProvider } from '@fluentui/react-shared-contexts';
@@ -8,6 +8,7 @@ import { MessageBarTitle } from '../MessageBarTitle/MessageBarTitle';
 import { MessageBarActions } from '../MessageBarActions/MessageBarActions';
 import { resetIdsForTests } from '@fluentui/react-utilities';
 import type { MessageBarProps } from './MessageBar.types';
+import { messageBarClassNames } from './useMessageBarStyles.styles';
 
 describe('MessageBar', () => {
   beforeAll(() => {
@@ -92,4 +93,118 @@ describe('MessageBar', () => {
     expect(announce).toHaveBeenCalledTimes(1);
     expect(announce).toHaveBeenCalledWith('TitleBody,Action 1Action 2', expect.anything());
   });
+
+  describe('should not flicker during reflow', () => {
+    let originalResizeObserver: typeof ResizeObserver;
+    let resizeCallback: ResizeObserverCallback | undefined;
+    let observedElement: HTMLElement | undefined;
+
+    beforeAll(() => {
+      originalResizeObserver = global.ResizeObserver;
+      global.ResizeObserver = class MockResizeObserver {
+        constructor(cb: ResizeObserverCallback) {
+          resizeCallback = cb;
+        }
+        public observe(el: Element) {
+          observedElement = el as HTMLElement;
+          Object.defineProperty(observedElement, 'scrollWidth', {
+            configurable: true,
+            get() {
+              return 640;
+            },
+          });
+        }
+        public unobserve() {
+          /* noop */
+        }
+        public disconnect() {
+          /* noop */
+        }
+      } as unknown as typeof ResizeObserver;
+    });
+
+    afterAll(() => {
+      global.ResizeObserver = originalResizeObserver;
+    });
+
+    beforeEach(() => {
+      resizeCallback = undefined;
+      observedElement = undefined;
+    });
+
+    const renderMessageBar = () =>
+      render(
+        <MessageBar>
+          <MessageBarBody>
+            <MessageBarTitle>Title</MessageBarTitle>
+            This message bar body is long enough that it needs to reflow to multiple lines.
+          </MessageBarBody>
+        </MessageBar>,
+      );
+
+    const isReflowing = (container: HTMLElement) =>
+      container.querySelector(`.${messageBarClassNames.bottomReflowSpacer}`) !== null;
+
+    // Simulate the container being resized (e.g. dragging the window / page) to a given width.
+    const resizeTo = (inlineSize: number) =>
+      act(() => {
+        resizeCallback?.(
+          [
+            {
+              target: observedElement,
+              borderBoxSize: [{ inlineSize, blockSize: 0 }],
+            } as unknown as ResizeObserverEntry,
+          ],
+          {} as ResizeObserver,
+        );
+      });
+
+    it('reflows to multiline when the container is narrower than the content', () => {
+      const { container } = renderMessageBar();
+      const singleLineWidth = observedElement!.scrollWidth;
+
+      // Wider than the content - stays single line.
+      resizeTo(singleLineWidth + 100);
+      expect(isReflowing(container)).toBe(false);
+
+      // Narrower than the content - reflows to multiline.
+      resizeTo(singleLineWidth - 100);
+      expect(isReflowing(container)).toBe(true);
+    });
+
+    it('does not flicker while the container width changes during a drag resize', () => {
+      const { container } = renderMessageBar();
+      const singleLineWidth = observedElement!.scrollWidth;
+
+      // Narrow enough to reflow.
+      resizeTo(singleLineWidth - 100);
+      expect(isReflowing(container)).toBe(true);
+
+      // Simulate a drag where the width changes but always stays below the width of the single line layout.
+      const observed: boolean[] = [];
+      for (const width of [singleLineWidth - 120, singleLineWidth - 40, singleLineWidth - 80, singleLineWidth - 10]) {
+        resizeTo(width);
+        observed.push(isReflowing(container));
+      }
+
+      // It must stay multiline the entire time - no toggling back to single line.
+      expect(observed).toEqual([true, true, true, true]);
+    });
+
+    it('returns to single line only once there is room for the content again', () => {
+      const { container } = renderMessageBar();
+      const singleLineWidth = observedElement!.scrollWidth;
+
+      resizeTo(singleLineWidth - 100);
+      expect(isReflowing(container)).toBe(true);
+
+      // Growing but still not enough room - stays reflowed.
+      resizeTo(singleLineWidth - 1);
+      expect(isReflowing(container)).toBe(true);
+
+      // Enough room for the single line layout again - returns to single line.
+      resizeTo(singleLineWidth);
+      expect(isReflowing(container)).toBe(false);
+    });
+  });
 });
diff --git a/packages/react-components/react-message-bar/library/src/components/MessageBar/useMessageBarReflow.ts b/packages/react-components/react-message-bar/library/src/components/MessageBar/useMessageBarReflow.ts
index b8272479c1ccc2..17f9e7a880ab28 100644
--- a/packages/react-components/react-message-bar/library/src/components/MessageBar/useMessageBarReflow.ts
+++ b/packages/react-components/react-message-bar/library/src/components/MessageBar/useMessageBarReflow.ts
@@ -14,7 +14,7 @@ export function useMessageBarReflow(enabled: boolean = false): {
   // TODO: exclude types from this lint rule: https://github.com/microsoft/fluentui/issues/31286
 
   const resizeObserverRef = React.useRef<ResizeObserver | null>(null);
-  const prevInlineSizeRef = React.useRef(-1);
+  const singleLineWidthRef = React.useRef(Number.POSITIVE_INFINITY);
 
   const handleResize: ResizeObserverCallback = React.useCallback(
     entries => {
@@ -47,21 +47,18 @@ export function useMessageBarReflow(enabled: boolean = false): {
 
       let nextReflowing: boolean | undefined;
 
-      // No easy way to really determine when the single line layout will fit
-      // Just keep try to set single line layout as long as the size is growing
-      // Will cause flickering when size is being adjusted gradually (i.e. drag) - but this should not be a common case
       if (reflowingRef.current) {
-        if (prevInlineSizeRef.current < inlineSize) {
+        if (inlineSize >= singleLineWidthRef.current) {
           nextReflowing = false;
         }
       } else {
         const scrollWidth = target.scrollWidth;
         if (inlineSize < scrollWidth) {
           nextReflowing = true;
+          singleLineWidthRef.current = scrollWidth;
         }
       }
 
-      prevInlineSizeRef.current = inlineSize;
       if (typeof nextReflowing !== 'undefined' && reflowingRef.current !== nextReflowing) {
         reflowingRef.current = nextReflowing;
         forceUpdate();