diff --git a/packages/react/src/combobox/positioner/ComboboxPositioner.test.tsx b/packages/react/src/combobox/positioner/ComboboxPositioner.test.tsx
index 4648b2bf42f..2b2d71bf071 100644
--- a/packages/react/src/combobox/positioner/ComboboxPositioner.test.tsx
+++ b/packages/react/src/combobox/positioner/ComboboxPositioner.test.tsx
@@ -1,5 +1,6 @@
-import { expect } from 'vitest';
+import { expect, vi } from 'vitest';
 import * as React from 'react';
+import * as ReactDOMClient from 'react-dom/client';
 import { waitFor } from '@mui/internal-test-utils';
 import { Combobox } from '@base-ui/react/combobox';
 import { createRenderer, describeConformance, isJSDOM } from '#test-utils';
@@ -7,6 +8,61 @@ import { createRenderer, describeConformance, isJSDOM } from '#test-utils';
 describe('<Combobox.Positioner />', () => {
   const { render } = createRenderer();
 
+  it('should not lock body scroll when controlled value={[]} triggers a re-render', async () => {
+    // Render outside of act() to match real browser behavior where
+    // the initial render and the useEffect re-render are separate commits.
+    // Using act() batches them, hiding the bug.
+    const consoleSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+    const container = document.createElement('div');
+    document.body.appendChild(container);
+    document.body.removeAttribute('style');
+    document.documentElement.removeAttribute('style');
+
+    function Test() {
+      const [, forceRender] = React.useState(0);
+      React.useEffect(() => {
+        forceRender(1);
+      }, []);
+
+      return (
+        <Combobox.Root multiple value={[]}>
+          <Combobox.Input />
+          <Combobox.Portal>
+            <Combobox.Positioner />
+          </Combobox.Portal>
+        </Combobox.Root>
+      );
+    }
+
+    const root = ReactDOMClient.createRoot(container);
+    root.render(<Test />);
+
+    // Wait for mount + useEffect re-render + setTimeout(0) in ScrollLocker.acquire
+    await new Promise((resolve) => {
+      setTimeout(resolve, 500);
+    });
+
+    // Bug: the re-render causes forceMount, mounting the Positioner.
+    // The Positioner's `open` is `undefined` (not `false`), so
+    // `open && modal` evaluates to `undefined`, which triggers
+    // useScrollLock's default parameter `enabled = true`.
+    const bodyOverflowX = document.body.style.overflowX;
+    const bodyOverflowY = document.body.style.overflowY;
+    const htmlOverflowX = document.documentElement.style.overflowX;
+    const htmlOverflowY = document.documentElement.style.overflowY;
+
+    root.unmount();
+    container.remove();
+    document.body.removeAttribute('style');
+    document.documentElement.removeAttribute('style');
+    consoleSpy.mockRestore();
+
+    expect(bodyOverflowX).not.toBe('hidden');
+    expect(bodyOverflowY).not.toBe('hidden');
+    expect(htmlOverflowX).not.toBe('hidden');
+    expect(htmlOverflowY).not.toBe('hidden');
+  });
+
   describeConformance(<Combobox.Positioner />, () => ({
     refInstanceof: window.HTMLDivElement,
     render(node) {
diff --git a/packages/react/src/combobox/root/AriaCombobox.tsx b/packages/react/src/combobox/root/AriaCombobox.tsx
index 14cb4ef80fe..fd040bbb83f 100644
--- a/packages/react/src/combobox/root/AriaCombobox.tsx
+++ b/packages/react/src/combobox/root/AriaCombobox.tsx
@@ -84,6 +84,8 @@ export function AriaCombobox<Value = any, Mode extends SelectionMode = 'none'>(
     onSelectedValueChange,
     defaultInputValue: defaultInputValueProp,
     inputValue: inputValueProp,
+    open: openProp,
+    defaultOpen = false,
     selectionMode = 'none',
     onItemHighlighted: onItemHighlightedProp,
     name: nameProp,
@@ -219,8 +221,8 @@ export function AriaCombobox<Value = any, Mode extends SelectionMode = 'none'>(
   });
 
   const [open, setOpenUnwrapped] = useControlled({
-    controlled: props.open,
-    default: props.defaultOpen,
+    controlled: openProp,
+    default: defaultOpen,
     name: 'Combobox',
     state: 'open',
   });