diff --git a/frontend/src/lib/utils/groupDocuments.spec.ts b/frontend/src/lib/utils/groupDocuments.spec.ts
new file mode 100644
index 00000000..895a4d99
--- /dev/null
+++ b/frontend/src/lib/utils/groupDocuments.spec.ts
@@ -0,0 +1,165 @@
+import { describe, expect, it } from 'vitest';
+import { groupDocuments } from './groupDocuments';
+
+type Doc = {
+	id: string;
+	documentDate?: string | null;
+	sender?: { displayName: string } | null;
+	receivers?: { displayName: string }[];
+};
+
+const doc = (overrides: Partial<Doc> & { id: string }): Doc => ({
+	documentDate: null,
+	sender: null,
+	receivers: [],
+	...overrides
+});
+
+// ─── DATE sort ───────────────────────────────────────────────────────────────
+
+describe('groupDocuments — DATE sort', () => {
+	it('produces one group per distinct year', () => {
+		const docs = [
+			doc({ id: 'a', documentDate: '1923-04-12' }),
+			doc({ id: 'b', documentDate: '1938-01-01' }),
+			doc({ id: 'c', documentDate: '1965-08-03' })
+		];
+		const groups = groupDocuments(docs, 'DATE', 'Undatiert');
+		expect(groups.map((g) => g.label)).toEqual(['1923', '1938', '1965']);
+		expect(groups.every((g) => g.documents.length === 1)).toBe(true);
+	});
+
+	it('puts multiple docs from the same year into one group', () => {
+		const docs = [
+			doc({ id: 'a', documentDate: '1938-03-01' }),
+			doc({ id: 'b', documentDate: '1938-11-15' })
+		];
+		const groups = groupDocuments(docs, 'DATE', 'Undatiert');
+		expect(groups).toHaveLength(1);
+		expect(groups[0].label).toBe('1938');
+		expect(groups[0].documents).toHaveLength(2);
+	});
+
+	it('places undated docs in the fallback group at the bottom', () => {
+		const docs = [
+			doc({ id: 'a', documentDate: '1938-01-01' }),
+			doc({ id: 'b', documentDate: null }),
+			doc({ id: 'c', documentDate: null })
+		];
+		const groups = groupDocuments(docs, 'DATE', 'Undatiert');
+		expect(groups).toHaveLength(2);
+		expect(groups[0].label).toBe('1938');
+		expect(groups[1].label).toBe('Undatiert');
+		expect(groups[1].documents.map((d) => d.id)).toEqual(['b', 'c']);
+	});
+
+	it('returns one group with fallback label when all docs are undated', () => {
+		const docs = [doc({ id: 'a' }), doc({ id: 'b' })];
+		const groups = groupDocuments(docs, 'DATE', 'Undatiert');
+		expect(groups).toHaveLength(1);
+		expect(groups[0].label).toBe('Undatiert');
+	});
+
+	it('returns one group when all docs are from the same year', () => {
+		const docs = [
+			doc({ id: 'a', documentDate: '1938-01-01' }),
+			doc({ id: 'b', documentDate: '1938-06-15' })
+		];
+		const groups = groupDocuments(docs, 'DATE', 'Undatiert');
+		expect(groups).toHaveLength(1);
+	});
+});
+
+// ─── SENDER sort ─────────────────────────────────────────────────────────────
+
+describe('groupDocuments — SENDER sort', () => {
+	it('produces one group per distinct sender', () => {
+		const docs = [
+			doc({ id: 'a', sender: { displayName: 'Anna Müller' } }),
+			doc({ id: 'b', sender: { displayName: 'Karl Bauer' } }),
+			doc({ id: 'c', sender: { displayName: 'Anna Müller' } })
+		];
+		const groups = groupDocuments(docs, 'SENDER', 'Unbekannt');
+		expect(groups.map((g) => g.label)).toEqual(['Anna Müller', 'Karl Bauer']);
+		expect(groups[0].documents).toHaveLength(2);
+		expect(groups[1].documents).toHaveLength(1);
+	});
+
+	it('places docs with no sender in the fallback group at the bottom', () => {
+		const docs = [
+			doc({ id: 'a', sender: { displayName: 'Anna Müller' } }),
+			doc({ id: 'b', sender: null })
+		];
+		const groups = groupDocuments(docs, 'SENDER', 'Unbekannt');
+		expect(groups).toHaveLength(2);
+		expect(groups[0].label).toBe('Anna Müller');
+		expect(groups[1].label).toBe('Unbekannt');
+		expect(groups[1].documents[0].id).toBe('b');
+	});
+});
+
+// ─── RECEIVER sort ───────────────────────────────────────────────────────────
+
+describe('groupDocuments — RECEIVER sort', () => {
+	it('a doc with two receivers appears in both receiver groups', () => {
+		const docs = [
+			doc({
+				id: 'a',
+				receivers: [{ displayName: 'Anna' }, { displayName: 'Karl' }]
+			})
+		];
+		const groups = groupDocuments(docs, 'RECEIVER', 'Unbekannt');
+		expect(groups.map((g) => g.label)).toEqual(['Anna', 'Karl']);
+		expect(groups[0].documents[0].id).toBe('a');
+		expect(groups[1].documents[0].id).toBe('a');
+	});
+
+	it('places docs with no receivers in the fallback group at the bottom', () => {
+		const docs = [
+			doc({ id: 'a', receivers: [{ displayName: 'Anna' }] }),
+			doc({ id: 'b', receivers: [] })
+		];
+		const groups = groupDocuments(docs, 'RECEIVER', 'Unbekannt');
+		expect(groups).toHaveLength(2);
+		expect(groups[0].label).toBe('Anna');
+		expect(groups[1].label).toBe('Unbekannt');
+		expect(groups[1].documents[0].id).toBe('b');
+	});
+
+	it('composite keys are unique: groupLabel + doc.id identifies each item', () => {
+		const docs = [
+			doc({ id: 'a', receivers: [{ displayName: 'Anna' }, { displayName: 'Karl' }] }),
+			doc({ id: 'b', receivers: [{ displayName: 'Anna' }] })
+		];
+		const groups = groupDocuments(docs, 'RECEIVER', 'Unbekannt');
+		const keys = groups.flatMap((g) => g.documents.map((d) => `${g.label}-${d.id}`));
+		const uniqueKeys = new Set(keys);
+		expect(uniqueKeys.size).toBe(keys.length);
+	});
+});
+
+// ─── Non-groupable sorts ──────────────────────────────────────────────────────
+
+describe('groupDocuments — non-groupable sorts', () => {
+	it('TITLE sort returns one group containing all documents', () => {
+		const docs = [doc({ id: 'a' }), doc({ id: 'b' })];
+		const groups = groupDocuments(docs, 'TITLE', 'Undatiert');
+		expect(groups).toHaveLength(1);
+		expect(groups[0].documents).toHaveLength(2);
+	});
+
+	it('UPLOAD_DATE sort returns one group containing all documents', () => {
+		const docs = [doc({ id: 'a' }), doc({ id: 'b' })];
+		const groups = groupDocuments(docs, 'UPLOAD_DATE', 'Undatiert');
+		expect(groups).toHaveLength(1);
+		expect(groups[0].documents).toHaveLength(2);
+	});
+});
+
+// ─── Edge cases ──────────────────────────────────────────────────────────────
+
+describe('groupDocuments — edge cases', () => {
+	it('returns empty array for an empty document list', () => {
+		expect(groupDocuments([], 'DATE', 'Undatiert')).toEqual([]);
+	});
+});
diff --git a/frontend/src/lib/utils/groupDocuments.ts b/frontend/src/lib/utils/groupDocuments.ts
new file mode 100644
index 00000000..d1926415
--- /dev/null
+++ b/frontend/src/lib/utils/groupDocuments.ts
@@ -0,0 +1,56 @@
+export type GroupableDoc = {
+	id: string;
+	documentDate?: string | null;
+	sender?: { displayName: string } | null;
+	receivers?: { displayName: string }[];
+};
+
+export type DocumentGroup<T extends GroupableDoc> = {
+	label: string;
+	documents: T[];
+};
+
+const GROUPABLE_SORTS = ['DATE', 'SENDER', 'RECEIVER'] as const;
+type GroupableSort = (typeof GROUPABLE_SORTS)[number];
+
+export function groupDocuments<T extends GroupableDoc>(
+	docs: T[],
+	sort: string,
+	fallbackLabel: string
+): DocumentGroup<T>[] {
+	if (docs.length === 0) return [];
+	if (!GROUPABLE_SORTS.includes(sort as GroupableSort)) {
+		return [{ label: '', documents: [...docs] }];
+	}
+
+	const groupMap = new Map<string, T[]>();
+	const fallbackDocs: T[] = [];
+
+	for (const doc of docs) {
+		const keys = extractGroupKeys(doc, sort as GroupableSort);
+		if (keys.length === 0) {
+			fallbackDocs.push(doc);
+		} else {
+			for (const key of keys) {
+				if (!groupMap.has(key)) groupMap.set(key, []);
+				groupMap.get(key)!.push(doc);
+			}
+		}
+	}
+
+	const groups = [...groupMap.entries()].map(([label, documents]) => ({ label, documents }));
+	if (fallbackDocs.length > 0) groups.push({ label: fallbackLabel, documents: fallbackDocs });
+	return groups;
+}
+
+function extractGroupKeys<T extends GroupableDoc>(doc: T, sort: GroupableSort): string[] {
+	if (sort === 'DATE') {
+		const year = doc.documentDate
+			? String(new Date(doc.documentDate + 'T12:00:00').getFullYear())
+			: null;
+		return year ? [year] : [];
+	}
+	if (sort === 'SENDER') return doc.sender ? [doc.sender.displayName] : [];
+	if (sort === 'RECEIVER') return (doc.receivers ?? []).map((r) => r.displayName);
+	return [];
+}