diff --git a/.changeset/switch-atom.md b/.changeset/switch-atom.md
new file mode 100644
index 0000000..b4b63f1
--- /dev/null
+++ b/.changeset/switch-atom.md
@@ -0,0 +1,11 @@
+---
+'@code-sherpas/pharos-react': minor
+---
+
+Add `Switch`, a single on/off toggle (the shadcn Switch contract) over Base
+UI's `Switch` — `role="switch"`, Space to toggle, and the shared Button / Input
+/ Checkbox focus ring. Sibling to `Checkbox` in the boolean form-control
+family: same `checked` / `onCheckedChange` / `disabled` API, no third
+`indeterminate` state (a toggle is binary). Label-less by design (D11): pair
+with a `<label htmlFor>`; error via `aria-invalid`. Geometry is fully
+token-derived (40×24 track, 16px thumb, spacing-4 travel); no icon dependency.
diff --git a/NAMING-decisions.md b/NAMING-decisions.md
index 07ed9ca..545025c 100644
--- a/NAMING-decisions.md
+++ b/NAMING-decisions.md
@@ -1664,3 +1664,57 @@ Maps to the various native/`@headlessui` checkboxes across the forms
 (settings, filters, consent). Like the other form atoms, a like-for-like
 swap (prop remap + `<label>` wiring) is incremental; checkbox groups with
 custom layout/state are case-by-case.
+
+## Switch (D21, 2026-07-15)
+
+A single on/off toggle. Wraps Base UI's `Switch` — a `role="switch"` button
+plus a hidden form input, Space to toggle, and the shared focus ring. The
+second of the boolean form controls, completing the family opened by Checkbox
+(D20): Checkbox → **Switch** → Radio to follow.
+
+### Why a separate atom from Checkbox (canonical, not cosmetic)
+
+shadcn ships `Switch` and `Checkbox` as distinct components, and the split is
+semantic, not visual: a **Switch** takes effect immediately (a setting you
+flip — notifications on/off), while a **Checkbox** is a selection usually
+submitted with a surrounding form (accept terms, pick options). ARIA backs the
+distinction with separate roles (`switch` vs `checkbox`). Merging them into one
+configurable control would blur both contracts (Rule #0: don't force it).
+
+The API is deliberately the same shape as Checkbox — `checked` /
+`onCheckedChange` / `disabled`, label-less (Escuela 1, D11) with error via
+`aria-invalid`, no `size` axis in v1 (shadcn ships none). The one intentional
+difference: **no `indeterminate`**. A toggle is binary; there is no third
+state (Base UI's `Switch` exposes none either).
+
+### `role="switch"`, not a styled checkbox
+
+Base UI's `Switch` renders `role="switch"` with `aria-checked`, the canonical
+ARIA APG toggle semantics — not a `role="checkbox"` restyled to look like a
+toggle. Screen readers announce "switch, on/off", which is the correct mental
+model for an immediate setting.
+
+### Token-derived geometry (no hardcoded pixels)
+
+The track is 40×24 (`spacing-10` × `spacing-6`) with `spacing-1` (4px) padding
+around a 16px (`spacing-4`) thumb, so the thumb travels exactly `spacing-4`
+(32px inner − 16px thumb) — every dimension is a `--pharos-*` token
+(NON-NEGOTIABLE #1). The track is intentionally larger than the 20px Checkbox
+box: a toggle is a bigger hit target and reads as a distinct control shape.
+
+The **on** track uses `neutral-900` — the same fill as the Checkbox's checked
+state, keeping one "active control" color across the family. The **off** track
+is `neutral-300`; the perceivable state indicator is the thumb (white with a
+`neutral-500` hairline, ≈5.4:1), which stays distinguishable on both the light
+off track and the dark on track — satisfying WCAG 1.4.11 without a track
+border (which would break the clean token travel). Error state is an inset
+1px ring (`semantic.error`) so the track keeps its geometry. No icon
+dependency — the thumb is a plain `<span>` (unlike Checkbox's inline SVG
+marks).
+
+### Mapping from Alexandria
+
+Maps to the various boolean toggles / on-off settings across Alexandria's
+forms. Like the other form atoms, a like-for-like swap (prop remap +
+`<label>` wiring) is incremental; toggles wired into complex local state are
+case-by-case (Fase 6).
diff --git a/examples/saas-demo/e2e/settings.spec.ts b/examples/saas-demo/e2e/settings.spec.ts
index 24e9429..7761dec 100644
--- a/examples/saas-demo/e2e/settings.spec.ts
+++ b/examples/saas-demo/e2e/settings.spec.ts
@@ -69,4 +69,14 @@ test.describe('Settings form', () => {
     await page.getByText('Email me product updates').click();
     await expect(box).toHaveAttribute('aria-checked', 'true');
   });
+
+  test('toggles the weekly-digest switch', async ({ page }) => {
+    const toggle = page.getByRole('switch', { name: 'Send me a weekly digest' });
+    await expect(toggle).toHaveAttribute('aria-checked', 'false');
+    await toggle.click();
+    await expect(toggle).toHaveAttribute('aria-checked', 'true');
+    // Label click toggles it back (label/control association).
+    await page.getByText('Send me a weekly digest').click();
+    await expect(toggle).toHaveAttribute('aria-checked', 'false');
+  });
 });
diff --git a/examples/saas-demo/src/pages/Settings.tsx b/examples/saas-demo/src/pages/Settings.tsx
index 1b63cf7..4c19869 100644
--- a/examples/saas-demo/src/pages/Settings.tsx
+++ b/examples/saas-demo/src/pages/Settings.tsx
@@ -21,6 +21,7 @@ import {
   SelectTrigger,
   SelectValue,
   Separator,
+  Switch,
   Textarea,
 } from '@code-sherpas/pharos-react';
 import { SKILLS, TIMEZONES } from '../data';
@@ -38,6 +39,7 @@ export function SettingsPage() {
   const [visibility, setVisibility] = useState('organization');
   const [skills, setSkills] = useState<string[]>(['React']);
   const [notify, setNotify] = useState(true);
+  const [weeklyDigest, setWeeklyDigest] = useState(false);
   const [nameError, setNameError] = useState(false);
   const [saved, setSaved] = useState(false);
 
@@ -181,6 +183,20 @@ export function SettingsPage() {
           </label>
         </div>
 
+        <div className="field">
+          <label htmlFor="digest" style={{ display: 'inline-flex', alignItems: 'center', gap: 8 }}>
+            <Switch
+              id="digest"
+              checked={weeklyDigest}
+              onCheckedChange={(checked) => {
+                setWeeklyDigest(checked);
+                touch();
+              }}
+            />
+            Send me a weekly digest
+          </label>
+        </div>
+
         <Separator />
         <div className="settings-actions">
           <Button type="submit">Save changes</Button>
diff --git a/src/components/Switch.module.css b/src/components/Switch.module.css
new file mode 100644
index 0000000..7af084f
--- /dev/null
+++ b/src/components/Switch.module.css
@@ -0,0 +1,88 @@
+/* Switch.module.css
+ *
+ * Per-component CSS Modules definition (Decision D9). Hashed into
+ * `dist/styles.css`. Styles a single on/off toggle (the shadcn "Switch"
+ * contract) over Base UI's `Switch`. State is driven by Base UI's
+ * `data-checked` / `data-disabled` attributes.
+ *
+ * The atom owns no label (Escuela 1, D11): the consumer pairs it with a
+ * `<label htmlFor>`. Error state is conveyed with `aria-invalid`, like Input /
+ * Textarea / Checkbox. No icon dependency — the thumb is a plain `<span>`.
+ *
+ * Geometry is fully token-derived: a 40×24 track (spacing-10 × spacing-6) with
+ * spacing-1 (4px) padding houses a 16px (spacing-4) thumb, so the thumb travels
+ * exactly spacing-4 (32px inner − 16px thumb) with no hardcoded pixel value.
+ */
+
+.root {
+  box-sizing: border-box;
+  display: inline-flex;
+  align-items: center;
+  flex-shrink: 0;
+  width: var(--pharos-spacing-10);
+  height: var(--pharos-spacing-6);
+  padding: var(--pharos-spacing-1);
+
+  border: none;
+  border-radius: var(--pharos-radius-full);
+  /* Off track. The perceivable state indicator is the thumb (white with a
+   * neutral-500 hairline, ≈5.4:1) — it stays distinguishable on both the light
+   * off track and the dark on track, satisfying WCAG 1.4.11. */
+  background-color: var(--pharos-color-neutral-300);
+
+  cursor: pointer;
+  transition-property: background-color, box-shadow;
+  transition-duration: var(--pharos-duration-fast);
+  transition-timing-function: var(--pharos-easing-out);
+}
+
+.root:hover:not([data-disabled]):not([data-checked]) {
+  background-color: var(--pharos-color-neutral-400);
+}
+
+/* On track — the neutral-900 fill, matching the Checkbox's checked look. */
+.root[data-checked] {
+  background-color: var(--pharos-color-neutral-900);
+}
+
+.root[data-checked]:hover:not([data-disabled]) {
+  background-color: var(--pharos-color-neutral-700);
+}
+
+/* Error state via the standard aria-invalid attribute (D11). An inset ring so
+ * the track keeps its geometry (no border-driven layout shift on the thumb). */
+.root[aria-invalid='true'] {
+  box-shadow: inset 0 0 0 1px var(--pharos-color-semantic-error-fg);
+}
+
+.root:focus-visible {
+  outline: none;
+  /* Same two-stop ring as Button / Input / Checkbox so the system has one
+   * focus look. */
+  box-shadow:
+    0 0 0 2px var(--pharos-color-base-white),
+    0 0 0 4px var(--pharos-color-primary-600);
+}
+
+.root[data-disabled] {
+  cursor: not-allowed;
+  opacity: 0.5;
+}
+
+.thumb {
+  box-sizing: border-box;
+  width: var(--pharos-spacing-4);
+  height: var(--pharos-spacing-4);
+  border: 1px solid var(--pharos-color-neutral-500);
+  border-radius: var(--pharos-radius-full);
+  background-color: var(--pharos-color-base-white);
+
+  transition-property: transform;
+  transition-duration: var(--pharos-duration-fast);
+  transition-timing-function: var(--pharos-easing-out);
+}
+
+/* Slide to the far edge: inner width (32px) − thumb (16px) = spacing-4. */
+.root[data-checked] .thumb {
+  transform: translateX(var(--pharos-spacing-4));
+}
diff --git a/src/components/Switch.stories.tsx b/src/components/Switch.stories.tsx
new file mode 100644
index 0000000..00072a8
--- /dev/null
+++ b/src/components/Switch.stories.tsx
@@ -0,0 +1,47 @@
+import type { Meta, StoryObj } from '@storybook/react-vite';
+import { Switch } from './Switch';
+
+/**
+ * `Switch` is a single on/off toggle (Decision D21). It wraps Base UI's
+ * `Switch` — `role="switch"`, Space to toggle, focus ring shared with Button /
+ * Input / Checkbox. Sibling to `Checkbox` in the boolean form-control family;
+ * a Switch takes effect immediately (a setting you flip). The atom owns no
+ * label (Escuela 1, D11): pair it with a `<label htmlFor>`. Error state via
+ * `aria-invalid`.
+ */
+const meta: Meta<typeof Switch> = {
+  title: 'Components/Switch',
+  component: Switch,
+  // The raw <label> text inherits the DS base typography from the story canvas
+  // (#storybook-root, set in preview.css after #151) — no decorator needed.
+};
+
+export default meta;
+type Story = StoryObj<typeof meta>;
+
+/** The label pattern: a `<label htmlFor>` paired with the control. */
+export const WithLabel: Story = {
+  render: () => (
+    <label style={{ display: 'inline-flex', alignItems: 'center', gap: 8 }} htmlFor="notify">
+      <Switch id="notify" defaultChecked />
+      Enable notifications
+    </label>
+  ),
+};
+
+/** All resting states side by side (each labelled for a11y). */
+export const States: Story = {
+  render: () => (
+    <div style={{ display: 'flex', gap: 20, alignItems: 'center' }}>
+      <Switch aria-label="Off" />
+      <Switch aria-label="On" defaultChecked />
+      <Switch aria-label="Disabled" disabled />
+      <Switch aria-label="Disabled on" disabled defaultChecked />
+    </div>
+  ),
+};
+
+/** Error state via the standard `aria-invalid` attribute. */
+export const Invalid: Story = {
+  render: () => <Switch aria-label="Required toggle" aria-invalid />,
+};
diff --git a/src/components/Switch.tsx b/src/components/Switch.tsx
new file mode 100644
index 0000000..153402f
--- /dev/null
+++ b/src/components/Switch.tsx
@@ -0,0 +1,38 @@
+import type { ComponentProps } from 'react';
+import { Switch as BaseSwitch } from '@base-ui/react/switch';
+import { cn } from '../lib/cn';
+import styles from './Switch.module.css';
+
+/**
+ * A single on/off toggle control (the shadcn **Switch** contract). Wraps Base
+ * UI's `Switch`, which renders a `role="switch"` button plus a hidden form
+ * input, and carries the ARIA state (`aria-checked`), keyboard toggle (Space)
+ * and focus handling.
+ *
+ * Sibling to `Checkbox` (D20) in the boolean form-control family: same
+ * `checked` / `onCheckedChange` / `disabled` API, same shared focus ring. The
+ * difference is semantic — a Switch takes effect immediately (a setting you
+ * flip), a Checkbox is a selection that is usually submitted with a form. It
+ * has no third `indeterminate` state (a toggle is binary).
+ *
+ * Like `Input` / `Textarea` / `Checkbox` (Escuela 1, D11), the atom owns no
+ * label: pair it with a `<label htmlFor>`. Error state is conveyed via
+ * `aria-invalid`, not a custom prop.
+ *
+ * @example
+ * <label htmlFor="notify" className="flex items-center gap-2">
+ *   <Switch id="notify" checked={enabled} onCheckedChange={setEnabled} />
+ *   Enable notifications
+ * </label>
+ */
+export interface SwitchProps extends ComponentProps<typeof BaseSwitch.Root> {}
+
+export function Switch({ className, ...rest }: SwitchProps) {
+  return (
+    <BaseSwitch.Root data-pharos-slot="switch" className={cn(styles.root, className)} {...rest}>
+      <BaseSwitch.Thumb className={styles.thumb} />
+    </BaseSwitch.Root>
+  );
+}
+
+Switch.displayName = 'Switch';
diff --git a/src/index.ts b/src/index.ts
index f49432a..39be61a 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -173,4 +173,6 @@ export type {
 } from './components/Dialog';
 export { Checkbox } from './components/Checkbox';
 export type { CheckboxProps } from './components/Checkbox';
+export { Switch } from './components/Switch';
+export type { SwitchProps } from './components/Switch';
 export { cn } from './lib/cn';
diff --git a/tests/Switch.test.tsx b/tests/Switch.test.tsx
new file mode 100644
index 0000000..6c1fc9e
--- /dev/null
+++ b/tests/Switch.test.tsx
@@ -0,0 +1,81 @@
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { Switch } from '../src/components/Switch';
+
+describe('Switch', () => {
+  it('renders a switch, off by default', () => {
+    render(<Switch aria-label="Notifications" />);
+    const toggle = screen.getByRole('switch', { name: 'Notifications' });
+    expect(toggle).toBeInTheDocument();
+    expect(toggle).toHaveAttribute('aria-checked', 'false');
+  });
+
+  it('toggles checked state on click', async () => {
+    const user = userEvent.setup();
+    render(<Switch aria-label="Notifications" />);
+    const toggle = screen.getByRole('switch', { name: 'Notifications' });
+
+    await user.click(toggle);
+    expect(toggle).toHaveAttribute('aria-checked', 'true');
+    await user.click(toggle);
+    expect(toggle).toHaveAttribute('aria-checked', 'false');
+  });
+
+  it('calls onCheckedChange when toggled (controlled)', async () => {
+    const user = userEvent.setup();
+    const onCheckedChange = vi.fn();
+    render(<Switch aria-label="Notifications" checked={false} onCheckedChange={onCheckedChange} />);
+
+    await user.click(screen.getByRole('switch', { name: 'Notifications' }));
+    expect(onCheckedChange).toHaveBeenCalledWith(true, expect.anything());
+  });
+
+  it('toggles with the Space key', async () => {
+    const user = userEvent.setup();
+    render(<Switch aria-label="Notifications" />);
+    const toggle = screen.getByRole('switch', { name: 'Notifications' });
+
+    toggle.focus();
+    await user.keyboard(' ');
+    expect(toggle).toHaveAttribute('aria-checked', 'true');
+  });
+
+  it('does not toggle when disabled', async () => {
+    const user = userEvent.setup();
+    const onCheckedChange = vi.fn();
+    render(<Switch aria-label="Notifications" disabled onCheckedChange={onCheckedChange} />);
+
+    await user.click(screen.getByRole('switch', { name: 'Notifications' }));
+    expect(onCheckedChange).not.toHaveBeenCalled();
+  });
+
+  it('associates with a label via htmlFor/id and toggles on label click', async () => {
+    const user = userEvent.setup();
+    render(
+      <label htmlFor="notify">
+        <Switch id="notify" />
+        Enable notifications
+      </label>,
+    );
+    const toggle = screen.getByRole('switch', { name: 'Enable notifications' });
+    await user.click(screen.getByText('Enable notifications'));
+    expect(toggle).toHaveAttribute('aria-checked', 'true');
+  });
+
+  it('reflects aria-invalid for the error state', () => {
+    render(<Switch aria-label="Required toggle" aria-invalid />);
+    expect(screen.getByRole('switch', { name: 'Required toggle' })).toHaveAttribute(
+      'aria-invalid',
+      'true',
+    );
+  });
+
+  it('tags the control via data-pharos-slot', () => {
+    render(<Switch aria-label="Notifications" />);
+    expect(screen.getByRole('switch', { name: 'Notifications' })).toHaveAttribute(
+      'data-pharos-slot',
+      'switch',
+    );
+  });
+});