diff --git a/.changeset/anchored-overlay-detached-anchor-aria.md b/.changeset/anchored-overlay-detached-anchor-aria.md
new file mode 100644
index 00000000000..ffb66cd2c86
--- /dev/null
+++ b/.changeset/anchored-overlay-detached-anchor-aria.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+AnchoredOverlay and SelectPanel: add trigger ARIA state for detached anchors
diff --git a/packages/react/src/AnchoredOverlay/AnchoredOverlay.docs.json b/packages/react/src/AnchoredOverlay/AnchoredOverlay.docs.json
index 75fe3cf0487..51a111cc026 100644
--- a/packages/react/src/AnchoredOverlay/AnchoredOverlay.docs.json
+++ b/packages/react/src/AnchoredOverlay/AnchoredOverlay.docs.json
@@ -169,6 +169,13 @@
       "description": "If true, a close button will be displayed in the top right corner of the overlay when on fullscreen mode.",
       "defaultValue": "true"
     },
+    {
+      "name": "anchorHasPopup",
+      "type": "'true' | 'dialog' | 'menu' | 'listbox' | 'tree' | 'grid'",
+      "required": false,
+      "description": "Indicates the type of popup opened by the anchor. Defaults to `'true'`, which is equivalent to `'menu'` in ARIA.",
+      "defaultValue": "'true'"
+    },
     {
       "name": "closeButtonProps",
       "type": "Partial<ButtonProps>",
diff --git a/packages/react/src/AnchoredOverlay/AnchoredOverlay.test.tsx b/packages/react/src/AnchoredOverlay/AnchoredOverlay.test.tsx
index d5e6b4179bf..6aae1f41c2b 100644
--- a/packages/react/src/AnchoredOverlay/AnchoredOverlay.test.tsx
+++ b/packages/react/src/AnchoredOverlay/AnchoredOverlay.test.tsx
@@ -3,6 +3,7 @@ import {describe, expect, it, vi} from 'vitest'
 import {act, fireEvent, render} from '@testing-library/react'
 import userEvent from '@testing-library/user-event'
 import {AnchoredOverlay} from '../AnchoredOverlay'
+import type {AnchorHasPopup} from '../AnchoredOverlay'
 import {Button} from '../Button'
 import BaseStyles from '../BaseStyles'
 import type {AnchorPosition} from '@primer/behaviors'
@@ -245,6 +246,195 @@ describe.each([true, false])(
   },
 )
 
+describe('AnchoredOverlay anchor ARIA', () => {
+  function DetachedOverlayTestComponent({
+    initiallyOpen = false,
+    anchorHasPopup = 'true',
+    consumerHasPopup,
+  }: {
+    initiallyOpen?: boolean
+    anchorHasPopup?: AnchorHasPopup
+    consumerHasPopup?: AnchorHasPopup
+  }) {
+    const [open, setOpen] = useState(initiallyOpen)
+    const anchorRef = useRef<HTMLButtonElement>(null)
+
+    return (
+      <BaseStyles>
+        <button
+          type="button"
+          ref={anchorRef}
+          data-testid="detached-anchor"
+          onClick={() => setOpen(isOpen => !isOpen)}
+          {...(consumerHasPopup ? {'aria-haspopup': consumerHasPopup} : {})}
+        >
+          Detached anchor
+        </button>
+        <AnchoredOverlay
+          open={open}
+          onOpen={() => setOpen(true)}
+          onClose={() => setOpen(false)}
+          renderAnchor={null}
+          anchorRef={anchorRef}
+          anchorHasPopup={anchorHasPopup}
+        >
+          <button type="button">Focusable Child</button>
+        </AnchoredOverlay>
+      </BaseStyles>
+    )
+  }
+
+  it('sets aria-haspopup and aria-expanded on a detached anchor when closed', async () => {
+    const {baseElement} = render(<DetachedOverlayTestComponent />)
+    await act(async () => {})
+
+    const anchor = baseElement.querySelector('[data-testid="detached-anchor"]')
+    expect(anchor).toHaveAttribute('aria-haspopup', 'true')
+    expect(anchor).toHaveAttribute('aria-expanded', 'false')
+  })
+
+  it('reflects aria-expanded on a detached anchor when opened and closed', async () => {
+    const {baseElement} = render(<DetachedOverlayTestComponent />)
+    await act(async () => {})
+
+    const anchor = baseElement.querySelector('[data-testid="detached-anchor"]') as HTMLElement
+    expect(anchor).toHaveAttribute('aria-expanded', 'false')
+
+    await userEvent.click(anchor)
+    expect(anchor).toHaveAttribute('aria-expanded', 'true')
+
+    await userEvent.click(anchor)
+    expect(anchor).toHaveAttribute('aria-expanded', 'false')
+  })
+
+  it('does not overwrite aria-haspopup supplied by a detached anchor consumer', async () => {
+    const {baseElement} = render(<DetachedOverlayTestComponent consumerHasPopup="listbox" />)
+    await act(async () => {})
+
+    const anchor = baseElement.querySelector('[data-testid="detached-anchor"]')
+    expect(anchor).toHaveAttribute('aria-haspopup', 'listbox')
+    expect(anchor).toHaveAttribute('aria-expanded', 'false')
+  })
+
+  it('reflects anchorHasPopup on the rendered anchor path', () => {
+    const {baseElement} = render(
+      <BaseStyles>
+        <AnchoredOverlay
+          open={false}
+          onOpen={() => {}}
+          onClose={() => {}}
+          anchorHasPopup="dialog"
+          renderAnchor={props => (
+            <button type="button" {...props}>
+              Anchor Button
+            </button>
+          )}
+        >
+          <button type="button">Focusable Child</button>
+        </AnchoredOverlay>
+      </BaseStyles>,
+    )
+
+    expect(baseElement.querySelector('button')).toHaveAttribute('aria-haspopup', 'dialog')
+  })
+
+  it('does not leave aria-expanded="true" on a detached anchor after close', async () => {
+    const {baseElement} = render(<DetachedOverlayTestComponent initiallyOpen={true} />)
+    await act(async () => {})
+
+    const anchor = baseElement.querySelector('[data-testid="detached-anchor"]') as HTMLElement
+    expect(anchor).toHaveAttribute('aria-expanded', 'true')
+
+    await userEvent.click(anchor)
+    expect(anchor).not.toHaveAttribute('aria-expanded', 'true')
+  })
+
+  it('does not write ARIA to a non-interactive detached anchor (<div> or plain <input>)', async () => {
+    function NonInteractiveAnchorComponent({tag}: {tag: 'div' | 'input'}) {
+      const [open, setOpen] = useState(false)
+      const anchorRef = useRef<HTMLElement>(null)
+
+      return (
+        <BaseStyles>
+          {tag === 'div' ? (
+            <div
+              ref={anchorRef as React.RefObject<HTMLDivElement>}
+              data-testid="anchor"
+              onClick={() => setOpen(isOpen => !isOpen)}
+            >
+              Detached div anchor
+            </div>
+          ) : (
+            <input
+              ref={anchorRef as React.RefObject<HTMLInputElement>}
+              data-testid="anchor"
+              onClick={() => setOpen(isOpen => !isOpen)}
+              readOnly
+              value="Detached input anchor"
+            />
+          )}
+          <AnchoredOverlay
+            open={open}
+            onOpen={() => setOpen(true)}
+            onClose={() => setOpen(false)}
+            renderAnchor={null}
+            anchorRef={anchorRef}
+          >
+            <button type="button">Focusable Child</button>
+          </AnchoredOverlay>
+        </BaseStyles>
+      )
+    }
+
+    for (const tag of ['div', 'input'] as const) {
+      const {baseElement, unmount} = render(<NonInteractiveAnchorComponent tag={tag} />)
+      await act(async () => {})
+
+      const anchor = baseElement.querySelector('[data-testid="anchor"]')
+      expect(anchor).not.toHaveAttribute('aria-haspopup')
+      expect(anchor).not.toHaveAttribute('aria-expanded')
+      unmount()
+    }
+  })
+
+  it('writes ARIA to a detached anchor with a popup-supporting role (e.g. role="combobox")', async () => {
+    function ComboboxAnchorComponent() {
+      const [open, setOpen] = useState(false)
+      const anchorRef = useRef<HTMLInputElement>(null)
+
+      return (
+        <BaseStyles>
+          <input
+            ref={anchorRef}
+            data-testid="anchor"
+            role="combobox"
+            aria-controls="overlay"
+            readOnly
+            value="Combobox anchor"
+            onClick={() => setOpen(isOpen => !isOpen)}
+          />
+          <AnchoredOverlay
+            open={open}
+            onOpen={() => setOpen(true)}
+            onClose={() => setOpen(false)}
+            renderAnchor={null}
+            anchorRef={anchorRef}
+          >
+            <button type="button">Focusable Child</button>
+          </AnchoredOverlay>
+        </BaseStyles>
+      )
+    }
+
+    const {baseElement} = render(<ComboboxAnchorComponent />)
+    await act(async () => {})
+
+    const anchor = baseElement.querySelector('[data-testid="anchor"]')
+    expect(anchor).toHaveAttribute('aria-haspopup', 'true')
+    expect(anchor).toHaveAttribute('aria-expanded', 'false')
+  })
+})
+
 describe('AnchoredOverlay scroll/resize cascade', () => {
   it('does not re-render after close when window scrolls (closed-overlay listeners detached)', async () => {
     // Before the `enabled: open` fix, useAnchoredPosition kept its scroll
diff --git a/packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx b/packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx
index a0c9d6a1da8..3f340a2a36d 100644
--- a/packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx
+++ b/packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx
@@ -18,6 +18,44 @@ import {useFeatureFlag} from '../FeatureFlags'
 import {widthMap} from '../Overlay/constants'
 import {reactMajorVersion} from '../utils/environment'
 
+type AnchorHasPopup = Exclude<React.AriaAttributes['aria-haspopup'], boolean | 'false' | undefined>
+export type {AnchorHasPopup}
+
+// `aria-haspopup` and `aria-expanded` describe a button-like trigger that opens
+// a popup, and are only valid on elements whose role supports both. In
+// detached-anchor mode the consumer owns the anchor element, so guard the
+// imperative ARIA writes: if the ref points at an element that can't legally
+// carry them (e.g. a wrapper `<div>` or a plain `<input>`/textbox), writing them
+// produces invalid ARIA (axe `aria-allowed-attr`). In that case the consumer
+// should point the ref at the real trigger (or give it an appropriate role), and
+// we skip rather than emit a violation.
+const rolesSupportingPopupAria = new Set([
+  'application',
+  'button',
+  'combobox',
+  'gridcell',
+  'link',
+  'menuitem',
+  'menuitemcheckbox',
+  'menuitemradio',
+  'tab',
+  'treeitem',
+])
+
+function anchorSupportsPopupAria(node: HTMLElement): boolean {
+  const explicitRole = node.getAttribute('role')
+  if (explicitRole) return rolesSupportingPopupAria.has(explicitRole)
+  switch (node.tagName) {
+    case 'BUTTON':
+    case 'SUMMARY':
+      return true
+    case 'A':
+      return node.hasAttribute('href')
+    default:
+      return false
+  }
+}
+
 interface AnchoredOverlayPropsWithAnchor {
   /**
    * A custom function component used to render the anchor element.
@@ -116,6 +154,10 @@ interface AnchoredOverlayBaseProps extends Pick<OverlayProps, 'height' | 'width'
    * Optional prop to display a close button in the overlay.
    */
   displayCloseButton?: boolean
+  /**
+   * Indicates the type of popup opened by the anchor. Defaults to `'true'`, which is equivalent to `'menu'` in ARIA.
+   */
+  anchorHasPopup?: AnchorHasPopup
   /**
    * Props to be spread on the close button in the overlay.
    */
@@ -178,6 +220,7 @@ export const AnchoredOverlay: React.FC<React.PropsWithChildren<AnchoredOverlayPr
   preventOverflow = true,
   onPositionChange,
   displayCloseButton = true,
+  anchorHasPopup = 'true',
   closeButtonProps = defaultCloseButtonProps,
   renderAs = 'portal',
   cssAnchorPositioningSettings,
@@ -314,6 +357,41 @@ export const AnchoredOverlay: React.FC<React.PropsWithChildren<AnchoredOverlayPr
     }
   }, [cssAnchorPositioning, anchorElement, anchorName])
 
+  useEffect(() => {
+    // Read the node from the ref (available once committed) rather than the
+    // anchorElement state, which lags a render behind for a detached anchor and
+    // would leave the collapsed trigger without aria-haspopup until first opened.
+    // aria-haspopup is stable for the component's lifetime, so this effect is
+    // kept separate from `open` to avoid rewriting the attribute on every toggle.
+    const node = anchorRef.current ?? anchorElement
+    if (renderAnchor !== null || !node) return
+    if (!anchorSupportsPopupAria(node)) return // avoid invalid ARIA on non-interactive anchors
+    if (node.hasAttribute('aria-haspopup')) return // don't clobber a consumer value
+
+    node.setAttribute('aria-haspopup', anchorHasPopup)
+
+    return () => {
+      if (node.getAttribute('aria-haspopup') === anchorHasPopup) {
+        node.removeAttribute('aria-haspopup')
+      }
+    }
+  }, [renderAnchor, anchorRef, anchorElement, anchorHasPopup])
+
+  useEffect(() => {
+    const node = anchorRef.current ?? anchorElement
+    if (renderAnchor !== null || !node) return
+    if (!anchorSupportsPopupAria(node)) return // avoid invalid ARIA on non-interactive anchors
+
+    const expandedValue = String(open)
+    node.setAttribute('aria-expanded', expandedValue)
+
+    return () => {
+      if (node.getAttribute('aria-expanded') === expandedValue) {
+        node.removeAttribute('aria-expanded')
+      }
+    }
+  }, [renderAnchor, anchorRef, anchorElement, open])
+
   useEffect(() => {
     if (!cssAnchorPositioning || !anchorElement) return
 
@@ -408,7 +486,7 @@ export const AnchoredOverlay: React.FC<React.PropsWithChildren<AnchoredOverlayPr
         renderAnchor({
           ref: anchorRef,
           id: anchorId,
-          'aria-haspopup': 'true',
+          'aria-haspopup': anchorHasPopup,
           'aria-expanded': open,
           tabIndex: 0,
           onClick: onAnchorClick,
diff --git a/packages/react/src/AnchoredOverlay/index.ts b/packages/react/src/AnchoredOverlay/index.ts
index 0a89ba09da0..3acca71015f 100644
--- a/packages/react/src/AnchoredOverlay/index.ts
+++ b/packages/react/src/AnchoredOverlay/index.ts
@@ -1,2 +1,2 @@
 export {AnchoredOverlay} from './AnchoredOverlay'
-export type {AnchoredOverlayProps} from './AnchoredOverlay'
+export type {AnchoredOverlayProps, AnchorHasPopup} from './AnchoredOverlay'
diff --git a/packages/react/src/SelectPanel/SelectPanel.test.tsx b/packages/react/src/SelectPanel/SelectPanel.test.tsx
index 32a3ade1f74..2147fcb8d6d 100644
--- a/packages/react/src/SelectPanel/SelectPanel.test.tsx
+++ b/packages/react/src/SelectPanel/SelectPanel.test.tsx
@@ -114,6 +114,43 @@ for (const usingRemoveActiveDescendant of [false, true]) {
       expect(trigger).toHaveAttribute('aria-expanded', 'false')
     })
 
+    it('sets aria-haspopup="true" on a detached anchor', async () => {
+      function DetachedSelectPanel() {
+        const [selected, setSelected] = React.useState<SelectPanelProps['items']>([])
+        const [filter, setFilter] = React.useState('')
+        const [open, setOpen] = React.useState(false)
+        const anchorRef = React.useRef<HTMLButtonElement>(null)
+
+        return (
+          <>
+            <button type="button" ref={anchorRef} onClick={() => setOpen(isOpen => !isOpen)}>
+              Detached select anchor
+            </button>
+            <SelectPanel
+              title="test title"
+              items={items}
+              selected={selected}
+              onSelectedChange={setSelected}
+              filterValue={filter}
+              onFilterChange={setFilter}
+              open={open}
+              onOpenChange={setOpen}
+              renderAnchor={null}
+              anchorRef={anchorRef}
+            />
+          </>
+        )
+      }
+
+      render(<DetachedSelectPanel />)
+
+      const trigger = screen.getByRole('button', {name: 'Detached select anchor'})
+      await waitFor(() => {
+        expect(trigger).toHaveAttribute('aria-haspopup', 'true')
+      })
+      expect(trigger).toHaveAttribute('aria-expanded', 'false')
+    })
+
     it('should call onActiveDescendantChanged when using keyboard while focusing on an item', async () => {
       const user = userEvent.setup()
       // jest function