diff --git a/packages/headless/package.json b/packages/headless/package.json
index a116c370d8a..898606b24d0 100644
--- a/packages/headless/package.json
+++ b/packages/headless/package.json
@@ -33,6 +33,10 @@
       "import": "./dist/primitives/autocomplete/index.js",
       "types": "./dist/primitives/autocomplete/index.d.ts"
     },
+    "./collapsible": {
+      "import": "./dist/primitives/collapsible/index.js",
+      "types": "./dist/primitives/collapsible/index.d.ts"
+    },
     "./dialog": {
       "import": "./dist/primitives/dialog/index.js",
       "types": "./dist/primitives/dialog/index.d.ts"
diff --git a/packages/headless/src/primitives/collapsible/collapsible-context.ts b/packages/headless/src/primitives/collapsible/collapsible-context.ts
new file mode 100644
index 00000000000..bab45ad7cfa
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/collapsible-context.ts
@@ -0,0 +1,19 @@
+import { createContext, useContext } from 'react';
+
+export interface CollapsibleContextValue {
+  open: boolean;
+  setOpen: (v: boolean) => void;
+  disabled: boolean;
+  triggerId: string;
+  panelId: string;
+}
+
+export const CollapsibleContext = createContext<CollapsibleContextValue | null>(null);
+
+export function useCollapsibleContext() {
+  const ctx = useContext(CollapsibleContext);
+  if (!ctx) {
+    throw new Error('Collapsible compound components must be used within <Collapsible.Root>');
+  }
+  return ctx;
+}
diff --git a/packages/headless/src/primitives/collapsible/collapsible-panel.tsx b/packages/headless/src/primitives/collapsible/collapsible-panel.tsx
new file mode 100644
index 00000000000..727017bcb43
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/collapsible-panel.tsx
@@ -0,0 +1,79 @@
+'use client';
+
+import { type RefObject, useLayoutEffect, useRef, useState } from 'react';
+import { useTransition } from '../../hooks/use-transition';
+import { type ComponentProps, mergeProps, renderElement } from '../../utils/render-element';
+import { useCollapsibleContext } from './collapsible-context';
+
+export interface CollapsiblePanelProps extends ComponentProps<'div'> {}
+
+export function CollapsiblePanel(props: CollapsiblePanelProps) {
+  const { render, ...otherProps } = props;
+  const { open, triggerId, panelId } = useCollapsibleContext();
+
+  const panelRef = useRef<HTMLElement | null>(null);
+  const [height, setHeight] = useState<number | undefined>(undefined);
+  const [width, setWidth] = useState<number | undefined>(undefined);
+
+  const hasBeenClosed = useRef(false);
+  if (!open) hasBeenClosed.current = true;
+
+  const { mounted, transitionProps } = useTransition({
+    open,
+    ref: panelRef as RefObject<HTMLElement>,
+  });
+
+  useLayoutEffect(() => {
+    if (!mounted) return;
+
+    const panel = panelRef.current;
+    if (!panel) return;
+
+    const measure = () => {
+      setHeight(panel.scrollHeight);
+      setWidth(panel.scrollWidth);
+    };
+
+    measure();
+
+    const ro = new ResizeObserver(measure);
+    ro.observe(panel, { box: 'border-box' });
+
+    return () => ro.disconnect();
+  }, [mounted]);
+
+  const state = { open };
+
+  const effectiveTransitionProps = !hasBeenClosed.current
+    ? {
+        ...transitionProps,
+        'data-cl-starting-style': undefined,
+        style: undefined,
+      }
+    : transitionProps;
+
+  const defaultProps: Record<string, unknown> = {
+    'data-cl-slot': 'collapsible-panel',
+    id: panelId,
+    role: 'region' as const,
+    'aria-labelledby': triggerId,
+    ref: panelRef,
+    ...effectiveTransitionProps,
+    style: {
+      '--collapsible-panel-height': height != null ? `${height}px` : undefined,
+      '--collapsible-panel-width': width != null ? `${width}px` : undefined,
+      ...effectiveTransitionProps.style,
+    },
+  };
+
+  return renderElement({
+    defaultTagName: 'div',
+    render,
+    enabled: mounted,
+    state,
+    stateAttributesMapping: {
+      open: (v: boolean): Record<string, string> | null => (v ? { 'data-cl-open': '' } : { 'data-cl-closed': '' }),
+    },
+    props: mergeProps<'div'>(defaultProps, otherProps),
+  });
+}
diff --git a/packages/headless/src/primitives/collapsible/collapsible-root.tsx b/packages/headless/src/primitives/collapsible/collapsible-root.tsx
new file mode 100644
index 00000000000..095665c150c
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/collapsible-root.tsx
@@ -0,0 +1,55 @@
+'use client';
+
+import { type ReactNode, useId, useMemo } from 'react';
+import { useControllableState } from '../../hooks/use-controllable-state';
+import { type ComponentProps, mergeProps, renderElement } from '../../utils/render-element';
+import { CollapsibleContext, type CollapsibleContextValue } from './collapsible-context';
+
+export interface CollapsibleProps extends ComponentProps<'div'> {
+  /** Controlled open state. */
+  open?: boolean;
+  /** Initial open state (uncontrolled). */
+  defaultOpen?: boolean;
+  /** Called when open state changes. */
+  onOpenChange?: (open: boolean) => void;
+  /** Disable the collapsible. @default false */
+  disabled?: boolean;
+  children: ReactNode;
+}
+
+export function CollapsibleRoot(props: CollapsibleProps) {
+  const { render, open: openProp, defaultOpen, onOpenChange, disabled = false, children, ...otherProps } = props;
+
+  const [open, setOpen] = useControllableState(openProp, defaultOpen ?? false, onOpenChange);
+
+  const rootId = useId();
+  const triggerId = `${rootId}trigger`;
+  const panelId = `${rootId}panel`;
+
+  const contextValue = useMemo<CollapsibleContextValue>(
+    () => ({ open, setOpen, disabled, triggerId, panelId }),
+    [open, setOpen, disabled, triggerId, panelId],
+  );
+
+  const state = { open, disabled };
+
+  const defaultProps: Record<string, unknown> = {
+    'data-cl-slot': 'collapsible-root',
+    children,
+  };
+
+  return (
+    <CollapsibleContext.Provider value={contextValue}>
+      {renderElement({
+        defaultTagName: 'div',
+        render,
+        state,
+        stateAttributesMapping: {
+          open: (v: boolean): Record<string, string> | null => (v ? { 'data-cl-open': '' } : { 'data-cl-closed': '' }),
+          disabled: (v: boolean) => (v ? { 'data-cl-disabled': '' } : null),
+        },
+        props: mergeProps<'div'>(defaultProps, otherProps),
+      })}
+    </CollapsibleContext.Provider>
+  );
+}
diff --git a/packages/headless/src/primitives/collapsible/collapsible-trigger.tsx b/packages/headless/src/primitives/collapsible/collapsible-trigger.tsx
new file mode 100644
index 00000000000..75d53f0231f
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/collapsible-trigger.tsx
@@ -0,0 +1,36 @@
+'use client';
+
+import { type ComponentProps, mergeProps, renderElement } from '../../utils/render-element';
+import { useCollapsibleContext } from './collapsible-context';
+
+export interface CollapsibleTriggerProps extends ComponentProps<'button'> {}
+
+export function CollapsibleTrigger(props: CollapsibleTriggerProps) {
+  const { render, ...otherProps } = props;
+  const { open, setOpen, disabled, triggerId, panelId } = useCollapsibleContext();
+
+  const state = { open, disabled };
+
+  const defaultProps: Record<string, unknown> = {
+    'data-cl-slot': 'collapsible-trigger',
+    id: triggerId,
+    type: 'button' as const,
+    'aria-expanded': open,
+    'aria-controls': panelId,
+    'aria-disabled': disabled || undefined,
+    onClick: () => {
+      if (!disabled) setOpen(!open);
+    },
+  };
+
+  return renderElement({
+    defaultTagName: 'button',
+    render,
+    state,
+    stateAttributesMapping: {
+      open: (v: boolean): Record<string, string> | null => (v ? { 'data-cl-open': '' } : { 'data-cl-closed': '' }),
+      disabled: (v: boolean) => (v ? { 'data-cl-disabled': '' } : null),
+    },
+    props: mergeProps<'button'>(defaultProps, otherProps),
+  });
+}
diff --git a/packages/headless/src/primitives/collapsible/collapsible.test.tsx b/packages/headless/src/primitives/collapsible/collapsible.test.tsx
new file mode 100644
index 00000000000..848ed41cff4
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/collapsible.test.tsx
@@ -0,0 +1,190 @@
+import { cleanup, render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { axe } from '../../test-utils/axe';
+import { Collapsible } from './index';
+
+afterEach(() => cleanup());
+
+function renderCollapsible(props: Partial<React.ComponentProps<typeof Collapsible.Root>> = {}) {
+  return render(
+    <Collapsible.Root {...props}>
+      <Collapsible.Trigger>Toggle</Collapsible.Trigger>
+      <Collapsible.Panel>Content</Collapsible.Panel>
+    </Collapsible.Root>,
+  );
+}
+
+describe('Collapsible', () => {
+  describe('slot attributes', () => {
+    it('renders root with data-cl-slot', () => {
+      renderCollapsible();
+      expect(document.querySelector('[data-cl-slot="collapsible-root"]')).toBeInTheDocument();
+    });
+
+    it('renders trigger with data-cl-slot', () => {
+      renderCollapsible();
+      expect(document.querySelector('[data-cl-slot="collapsible-trigger"]')).toBeInTheDocument();
+    });
+
+    it('renders panel with data-cl-slot when open', () => {
+      renderCollapsible({ defaultOpen: true });
+      expect(document.querySelector('[data-cl-slot="collapsible-panel"]')).toBeInTheDocument();
+    });
+  });
+
+  describe('open/close', () => {
+    it('opens panel on trigger click', async () => {
+      const user = userEvent.setup();
+      renderCollapsible();
+
+      await user.click(screen.getByRole('button', { name: 'Toggle' }));
+
+      expect(screen.getByText('Content')).toBeInTheDocument();
+      expect(document.querySelector('[data-cl-slot="collapsible-root"]')).toHaveAttribute('data-cl-open', '');
+    });
+
+    it('closes panel on second trigger click', async () => {
+      const user = userEvent.setup();
+      renderCollapsible({ defaultOpen: true });
+
+      await user.click(screen.getByRole('button', { name: 'Toggle' }));
+
+      expect(document.querySelector('[data-cl-slot="collapsible-root"]')).toHaveAttribute('data-cl-closed', '');
+    });
+
+    it('calls onOpenChange when toggled', async () => {
+      const onOpenChange = vi.fn();
+      const user = userEvent.setup();
+      renderCollapsible({ onOpenChange });
+
+      await user.click(screen.getByRole('button', { name: 'Toggle' }));
+
+      expect(onOpenChange).toHaveBeenCalledWith(true);
+    });
+
+    it('starts closed by default', () => {
+      renderCollapsible();
+      expect(screen.queryByText('Content')).not.toBeInTheDocument();
+      expect(document.querySelector('[data-cl-slot="collapsible-root"]')).toHaveAttribute('data-cl-closed', '');
+    });
+
+    it('starts open with defaultOpen=true', () => {
+      renderCollapsible({ defaultOpen: true });
+      expect(screen.getByText('Content')).toBeInTheDocument();
+    });
+  });
+
+  describe('controlled value', () => {
+    it('respects controlled open prop', () => {
+      renderCollapsible({ open: true });
+      expect(screen.getByText('Content')).toBeInTheDocument();
+    });
+
+    it('does not change when controlled', async () => {
+      const user = userEvent.setup();
+      renderCollapsible({ open: false });
+
+      await user.click(screen.getByRole('button', { name: 'Toggle' }));
+
+      expect(screen.queryByText('Content')).not.toBeInTheDocument();
+    });
+  });
+
+  describe('ARIA attributes', () => {
+    it('trigger has aria-expanded=false when closed', () => {
+      renderCollapsible();
+      expect(screen.getByRole('button', { name: 'Toggle' })).toHaveAttribute('aria-expanded', 'false');
+    });
+
+    it('trigger has aria-expanded=true when open', () => {
+      renderCollapsible({ defaultOpen: true });
+      expect(screen.getByRole('button', { name: 'Toggle' })).toHaveAttribute('aria-expanded', 'true');
+    });
+
+    it('trigger has aria-controls linked to panel id', () => {
+      renderCollapsible({ defaultOpen: true });
+      const trigger = screen.getByRole('button', { name: 'Toggle' });
+      const panel = document.querySelector('[data-cl-slot="collapsible-panel"]');
+      expect(trigger).toHaveAttribute('aria-controls', panel?.getAttribute('id'));
+    });
+
+    it('panel has aria-labelledby linked to trigger id', () => {
+      renderCollapsible({ defaultOpen: true });
+      const trigger = screen.getByRole('button', { name: 'Toggle' });
+      const panel = document.querySelector('[data-cl-slot="collapsible-panel"]');
+      expect(panel).toHaveAttribute('aria-labelledby', trigger.getAttribute('id'));
+    });
+
+    it('panel has role=region', () => {
+      renderCollapsible({ defaultOpen: true });
+      expect(screen.getByRole('region')).toBeInTheDocument();
+    });
+  });
+
+  describe('animation lifecycle', () => {
+    it('panel is not in DOM when closed', () => {
+      renderCollapsible();
+      expect(document.querySelector('[data-cl-slot="collapsible-panel"]')).not.toBeInTheDocument();
+    });
+
+    it('applies data-cl-open on panel when open', () => {
+      renderCollapsible({ defaultOpen: true });
+      const panel = document.querySelector('[data-cl-slot="collapsible-panel"]');
+      expect(panel).toHaveAttribute('data-cl-open', '');
+    });
+
+    it('does not apply starting-style on initially open panel', () => {
+      renderCollapsible({ defaultOpen: true });
+      const panel = document.querySelector('[data-cl-slot="collapsible-panel"]');
+      expect(panel).not.toHaveAttribute('data-cl-starting-style');
+    });
+
+    it('sets --collapsible-panel-height CSS variable on panel', () => {
+      renderCollapsible({ defaultOpen: true });
+      const panel = document.querySelector('[data-cl-slot="collapsible-panel"]') as HTMLElement;
+      expect(panel.getAttribute('style')).toContain('--collapsible-panel-height');
+    });
+
+    it('sets --collapsible-panel-width CSS variable on panel', () => {
+      renderCollapsible({ defaultOpen: true });
+      const panel = document.querySelector('[data-cl-slot="collapsible-panel"]') as HTMLElement;
+      expect(panel.getAttribute('style')).toContain('--collapsible-panel-width');
+    });
+  });
+
+  describe('disabled', () => {
+    it('prevents toggle when disabled', async () => {
+      const onOpenChange = vi.fn();
+      const user = userEvent.setup();
+      renderCollapsible({ disabled: true, onOpenChange });
+
+      await user.click(screen.getByRole('button', { name: 'Toggle' }));
+
+      expect(onOpenChange).not.toHaveBeenCalled();
+    });
+
+    it('applies aria-disabled on trigger', () => {
+      renderCollapsible({ disabled: true });
+      expect(screen.getByRole('button', { name: 'Toggle' })).toHaveAttribute('aria-disabled', 'true');
+    });
+
+    it('applies data-cl-disabled on root and trigger', () => {
+      renderCollapsible({ disabled: true });
+      expect(document.querySelector('[data-cl-slot="collapsible-root"]')).toHaveAttribute('data-cl-disabled', '');
+      expect(document.querySelector('[data-cl-slot="collapsible-trigger"]')).toHaveAttribute('data-cl-disabled', '');
+    });
+  });
+
+  describe('accessibility (axe)', () => {
+    it('has no violations when closed', async () => {
+      const { container } = renderCollapsible();
+      expect(await axe(container)).toHaveNoViolations();
+    });
+
+    it('has no violations when open', async () => {
+      const { container } = renderCollapsible({ defaultOpen: true });
+      expect(await axe(container)).toHaveNoViolations();
+    });
+  });
+});
diff --git a/packages/headless/src/primitives/collapsible/index.ts b/packages/headless/src/primitives/collapsible/index.ts
new file mode 100644
index 00000000000..c1ad32a41fe
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/index.ts
@@ -0,0 +1,3 @@
+export * as Collapsible from './parts';
+
+export type { CollapsibleProps, CollapsibleTriggerProps, CollapsiblePanelProps } from './parts';
diff --git a/packages/headless/src/primitives/collapsible/parts.ts b/packages/headless/src/primitives/collapsible/parts.ts
new file mode 100644
index 00000000000..d09f50c23ac
--- /dev/null
+++ b/packages/headless/src/primitives/collapsible/parts.ts
@@ -0,0 +1,3 @@
+export { type CollapsibleProps, CollapsibleRoot as Root } from './collapsible-root';
+export { type CollapsibleTriggerProps, CollapsibleTrigger as Trigger } from './collapsible-trigger';
+export { type CollapsiblePanelProps, CollapsiblePanel as Panel } from './collapsible-panel';
diff --git a/packages/headless/vite.config.ts b/packages/headless/vite.config.ts
index 751450974d3..80ba5c893a5 100644
--- a/packages/headless/vite.config.ts
+++ b/packages/headless/vite.config.ts
@@ -18,6 +18,7 @@ export default defineConfig({
         'primitives/select/index': 'src/primitives/select/index.ts',
         'primitives/menu/index': 'src/primitives/menu/index.ts',
         'primitives/autocomplete/index': 'src/primitives/autocomplete/index.ts',
+        'primitives/collapsible/index': 'src/primitives/collapsible/index.ts',
         'primitives/dialog/index': 'src/primitives/dialog/index.ts',
         'utils/index': 'src/utils/index.ts',
         'hooks/use-controllable-state': 'src/hooks/use-controllable-state.ts',