diff --git a/frontend/src/lib/timeline/GroupingControl.svelte b/frontend/src/lib/timeline/GroupingControl.svelte
new file mode 100644
index 00000000..681c289d
--- /dev/null
+++ b/frontend/src/lib/timeline/GroupingControl.svelte
@@ -0,0 +1,114 @@
+<script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import type { GroupingMode } from './timelineGrouping';
+
+/**
+ * The Datum·Ereignis·Thema segmented control (#827, REQ-010/011/018). An ARIA radiogroup with
+ * roving tabindex — single selection, arrow-key navigable — deliberately distinct from #780's
+ * `aria-pressed` layer-filter toggles. Defaults to Datum. Each segment is ≥44×44px, carries a
+ * text label (full word as `aria-label`, an abbreviated label shown ≤360px so the control never
+ * overflows at 320px), and uses semantic tokens so the selected/unselected contrast holds in dark
+ * mode. When `disabled` (the Letters layer is off, nothing to regroup) the control stays in place
+ * — no reflow — keeps its `aria-checked` selection so re-enabling restores the mode, and announces
+ * a screen-reader reason.
+ */
+let {
+	mode = $bindable('date'),
+	disabled = false,
+	ariaLabel = m.timeline_grouping_aria_label()
+}: { mode?: GroupingMode; disabled?: boolean; ariaLabel?: string } = $props();
+
+interface Segment {
+	value: GroupingMode;
+	full: string;
+	short: string;
+}
+
+const segments: Segment[] = [
+	{
+		value: 'date',
+		full: m.timeline_grouping_segment_date(),
+		short: m.timeline_grouping_segment_date_short()
+	},
+	{
+		value: 'event',
+		full: m.timeline_grouping_segment_event(),
+		short: m.timeline_grouping_segment_event_short()
+	},
+	{
+		value: 'thema',
+		full: m.timeline_grouping_segment_thema(),
+		short: m.timeline_grouping_segment_thema_short()
+	}
+];
+
+function select(value: GroupingMode) {
+	if (disabled) return;
+	mode = value;
+}
+
+function onKeydown(event: KeyboardEvent) {
+	if (disabled) return;
+	const forward = event.key === 'ArrowRight' || event.key === 'ArrowDown';
+	const backward = event.key === 'ArrowLeft' || event.key === 'ArrowUp';
+	if (!forward && !backward) return;
+	event.preventDefault();
+	const index = segments.findIndex((s) => s.value === mode);
+	const delta = forward ? 1 : -1;
+	const next = segments[(index + delta + segments.length) % segments.length];
+	mode = next.value;
+	const groupEl = event.currentTarget as HTMLElement;
+	groupEl.querySelector<HTMLElement>(`[data-value="${next.value}"]`)?.focus();
+}
+</script>
+
+<div
+	role="radiogroup"
+	tabindex="-1"
+	aria-label={ariaLabel}
+	aria-disabled={disabled}
+	data-testid="grouping-control"
+	class="inline-flex overflow-hidden rounded-md border border-line"
+	onkeydown={onKeydown}
+>
+	{#each segments as segment (segment.value)}
+		<button
+			type="button"
+			role="radio"
+			data-value={segment.value}
+			aria-label={segment.full}
+			aria-checked={mode === segment.value}
+			tabindex={mode === segment.value ? 0 : -1}
+			disabled={disabled}
+			onclick={() => select(segment.value)}
+			style="display: inline-flex; align-items: center; justify-content: center; min-height: 44px; min-width: 44px"
+			class="px-3 py-2 font-sans text-xs font-semibold focus:outline-none focus-visible:ring-2 focus-visible:ring-brand-navy focus-visible:ring-inset"
+			class:bg-brand-navy={mode === segment.value && !disabled}
+			class:text-white={mode === segment.value && !disabled}
+			class:bg-surface={mode !== segment.value || disabled}
+			class:text-ink-3={mode !== segment.value || disabled}
+		>
+			<span class="seg-full">{segment.full}</span>
+			<span class="seg-short">{segment.short}</span>
+		</button>
+	{/each}
+</div>
+{#if disabled}
+	<span class="sr-only" role="status" data-testid="grouping-disabled-reason"
+		>{m.timeline_grouping_disabled_reason()}</span
+	>
+{/if}
+
+<style>
+.seg-short {
+	display: none;
+}
+@media (max-width: 360px) {
+	.seg-full {
+		display: none;
+	}
+	.seg-short {
+		display: inline;
+	}
+}
+</style>
diff --git a/frontend/src/lib/timeline/GroupingControl.svelte.spec.ts b/frontend/src/lib/timeline/GroupingControl.svelte.spec.ts
new file mode 100644
index 00000000..6516881d
--- /dev/null
+++ b/frontend/src/lib/timeline/GroupingControl.svelte.spec.ts
@@ -0,0 +1,106 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { tick } from 'svelte';
+import * as m from '$lib/paraglide/messages.js';
+import GroupingControl from './GroupingControl.svelte';
+
+afterEach(() => cleanup());
+
+const radios = () => Array.from(document.querySelectorAll('[role="radio"]')) as HTMLElement[];
+const group = () => document.querySelector('[role="radiogroup"]') as HTMLElement;
+const checkedValue = () =>
+	radios()
+		.find((r) => r.getAttribute('aria-checked') === 'true')
+		?.getAttribute('data-value');
+
+describe('GroupingControl (REQ-010)', () => {
+	it('renders three radios inside a radiogroup, each with aria-checked (a)', () => {
+		render(GroupingControl, {});
+		expect(group()).not.toBeNull();
+		const r = radios();
+		expect(r).toHaveLength(3);
+		r.forEach((radio) => expect(radio.hasAttribute('aria-checked')).toBe(true));
+	});
+
+	it('defaults to Datum (f)', () => {
+		render(GroupingControl, {});
+		expect(radios().filter((r) => r.getAttribute('aria-checked') === 'true')).toHaveLength(1);
+		expect(checkedValue()).toBe('date');
+	});
+
+	it('exposes a text label on every segment, not colour alone (d)', () => {
+		render(GroupingControl, {});
+		radios().forEach((r) => expect((r.textContent ?? '').trim().length).toBeGreaterThan(0));
+	});
+
+	it('gives the radiogroup an accessible name (e)', () => {
+		render(GroupingControl, {});
+		expect(group().getAttribute('aria-label')).toBe(m.timeline_grouping_aria_label());
+	});
+
+	it('each segment has a tap target of at least 44×44px (c)', () => {
+		render(GroupingControl, {});
+		radios().forEach((r) => {
+			const rect = r.getBoundingClientRect();
+			expect(rect.width).toBeGreaterThanOrEqual(44);
+			expect(rect.height).toBeGreaterThanOrEqual(44);
+		});
+	});
+
+	it('exposes each segment full word as an aria-label (REQ-011)', () => {
+		render(GroupingControl, {});
+		const labels = radios().map((r) => r.getAttribute('aria-label'));
+		expect(labels).toEqual([
+			m.timeline_grouping_segment_date(),
+			m.timeline_grouping_segment_event(),
+			m.timeline_grouping_segment_thema()
+		]);
+	});
+
+	it('moves the selection forward with the right arrow key (b)', async () => {
+		render(GroupingControl, { mode: 'date' });
+		group().dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowRight', bubbles: true }));
+		await tick();
+		expect(checkedValue()).toBe('event');
+	});
+
+	it('wraps to the last segment with the left arrow from Datum (b)', async () => {
+		render(GroupingControl, { mode: 'date' });
+		group().dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowLeft', bubbles: true }));
+		await tick();
+		expect(checkedValue()).toBe('thema');
+	});
+
+	it('selects a segment on click', async () => {
+		render(GroupingControl, { mode: 'date' });
+		const thema = radios().find((r) => r.getAttribute('data-value') === 'thema')!;
+		thema.click();
+		await tick();
+		expect(thema.getAttribute('aria-checked')).toBe('true');
+	});
+});
+
+describe('GroupingControl — disabled (REQ-018)', () => {
+	it('marks the radiogroup aria-disabled and keeps all radios in the DOM', () => {
+		render(GroupingControl, { mode: 'event', disabled: true });
+		expect(group().getAttribute('aria-disabled')).toBe('true');
+		expect(radios()).toHaveLength(3);
+	});
+
+	it('announces a screen-reader reason that letters are hidden', () => {
+		render(GroupingControl, { disabled: true });
+		const reason = document.querySelector('[data-testid="grouping-disabled-reason"]');
+		expect(reason?.textContent).toContain(m.timeline_grouping_disabled_reason());
+	});
+
+	it('retains the active mode while disabled (no reset to Datum)', () => {
+		render(GroupingControl, { mode: 'thema', disabled: true });
+		expect(checkedValue()).toBe('thema');
+	});
+
+	it('ignores arrow keys while disabled', () => {
+		render(GroupingControl, { mode: 'event', disabled: true });
+		group().dispatchEvent(new KeyboardEvent('keydown', { key: 'ArrowRight', bubbles: true }));
+		expect(checkedValue()).toBe('event');
+	});
+});