diff --git a/apps/admin/src/components/SessionForm.test.tsx b/apps/admin/src/components/SessionForm.test.tsx
new file mode 100644
index 0000000..6fe987c
--- /dev/null
+++ b/apps/admin/src/components/SessionForm.test.tsx
@@ -0,0 +1,198 @@
+import { render, screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import type { Activity, WorkSession } from '@solelog/shared';
+import SessionForm from './SessionForm';
+import { apiFetch } from '../lib/api';
+
+vi.mock('../lib/api', () => ({
+  apiFetch: vi.fn(),
+  ApiError: class ApiError extends Error {
+    status: number;
+    constructor(status: number, message: string) {
+      super(message);
+      this.status = status;
+      this.name = 'ApiError';
+    }
+  },
+}));
+
+const mockApiFetch = vi.mocked(apiFetch);
+
+const USERS = [
+  { id: 'u1', name: 'Jan', email: 'jan@example.com' },
+  { id: 'u2', name: 'Piet', email: 'piet@example.com' },
+];
+
+const ACTIVITIES: Activity[] = [
+  {
+    id: 10,
+    name: 'Frezen',
+    insole_types: ['Kurk', 'Berk', '3D'],
+    created_at: '2026-06-17T00:00:00.000Z',
+    sort_order: 0,
+  },
+  {
+    id: 11,
+    name: 'Lijmen',
+    insole_types: ['Kurk', 'Berk', '3D'],
+    created_at: '2026-06-17T00:00:00.000Z',
+    sort_order: 1,
+  },
+];
+
+function makeSession(over: Partial<WorkSession> = {}): WorkSession {
+  return {
+    id: 7,
+    user_id: 'u2',
+    activity_id: 11,
+    activity_name: 'Lijmen',
+    user_name: 'Piet',
+    insole_type: 'Berk',
+    pair_count: 3,
+    start_time: new Date('2026-06-17T08:00:00Z').toISOString(),
+    end_time: new Date('2026-06-17T09:00:00Z').toISOString(),
+    duration_seconds: 3000,
+    paused_seconds: 600,
+    paused_at: null,
+    status: 'completed',
+    source: 'manual',
+    notes: 'aantekening',
+    created_at: new Date('2026-06-17T09:00:00Z').toISOString(),
+    ...over,
+  };
+}
+
+// Route GET roster/activities to fixtures; POST/PUT resolve to a session.
+function mockEndpoints() {
+  mockApiFetch.mockImplementation((path: string) => {
+    if (path === '/api/admin/users') return Promise.resolve(USERS as never);
+    if (path === '/api/activities') return Promise.resolve(ACTIVITIES as never);
+    return Promise.resolve(makeSession() as never);
+  });
+}
+
+function renderForm(props: Parameters<typeof SessionForm>[0]) {
+  const queryClient = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(
+    <QueryClientProvider client={queryClient}>
+      <SessionForm {...props} />
+    </QueryClientProvider>,
+  );
+}
+
+describe('SessionForm', () => {
+  beforeEach(() => {
+    mockApiFetch.mockReset();
+    mockEndpoints();
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('create mode shows the worker picker and posts CreateManualSessionInput', async () => {
+    const onClose = vi.fn();
+    renderForm({ mode: 'create', onClose });
+
+    // Worker picker populated from the roster.
+    const worker = await screen.findByLabelText('Medewerker');
+    expect(worker).toBeInTheDocument();
+    await screen.findByRole('option', { name: 'Jan' });
+
+    await userEvent.selectOptions(worker, 'u1');
+    await userEvent.selectOptions(screen.getByLabelText('Handeling'), '10');
+    await userEvent.selectOptions(screen.getByLabelText('Zooltype'), 'Kurk');
+
+    // Times: 1h apart.
+    const start = screen.getByLabelText('Starttijd');
+    const end = screen.getByLabelText('Eindtijd');
+    await userEvent.clear(start);
+    await userEvent.type(start, '2026-06-17T08:00');
+    await userEvent.clear(end);
+    await userEvent.type(end, '2026-06-17T09:00');
+
+    await userEvent.click(screen.getByRole('button', { name: /Opslaan/ }));
+
+    await waitFor(() =>
+      expect(mockApiFetch).toHaveBeenCalledWith(
+        '/api/admin/sessions',
+        expect.objectContaining({ method: 'POST' }),
+      ),
+    );
+
+    const call = mockApiFetch.mock.calls.find((c) => c[0] === '/api/admin/sessions');
+    expect(call).toBeDefined();
+    const body = JSON.parse((call![1] as RequestInit).body as string);
+    expect(body).toMatchObject({
+      user_id: 'u1',
+      activity_id: 10,
+      insole_type: 'Kurk',
+    });
+    // ISO times built from the datetime-local values.
+    expect(new Date(body.end_time).getTime() - new Date(body.start_time).getTime()).toBe(3_600_000);
+    expect(onClose).toHaveBeenCalled();
+  });
+
+  it('edit mode hides the worker picker, prefills, and PUTs the changes', async () => {
+    const onClose = vi.fn();
+    renderForm({ mode: 'edit', session: makeSession(), onClose });
+
+    // No worker picker in edit mode.
+    await screen.findByLabelText('Handeling');
+    expect(screen.queryByLabelText('Medewerker')).not.toBeInTheDocument();
+
+    // Prefilled: activity 11 (Lijmen) — wait for the roster/activities to load.
+    await screen.findByRole('option', { name: 'Lijmen' });
+    expect((screen.getByLabelText('Handeling') as HTMLSelectElement).value).toBe('11');
+    expect((screen.getByLabelText('Notitie') as HTMLTextAreaElement).value).toBe('aantekening');
+
+    // Status select is present only in edit mode.
+    expect(screen.getByLabelText('Status')).toBeInTheDocument();
+
+    await userEvent.click(screen.getByRole('button', { name: /Opslaan/ }));
+
+    await waitFor(() =>
+      expect(mockApiFetch).toHaveBeenCalledWith(
+        '/api/admin/sessions/7',
+        expect.objectContaining({ method: 'PUT' }),
+      ),
+    );
+
+    const call = mockApiFetch.mock.calls.find((c) => c[0] === '/api/admin/sessions/7');
+    const body = JSON.parse((call![1] as RequestInit).body as string);
+    expect(body).toMatchObject({
+      activity_id: 11,
+      insole_type: 'Berk',
+      pair_count: 3,
+      status: 'completed',
+      notes: 'aantekening',
+    });
+    expect(onClose).toHaveBeenCalled();
+  });
+
+  it('shows the gewerkt preview = end - start - paused', async () => {
+    // 1h span (3600s) minus 600s paused = 3000s = 00:50:00.
+    renderForm({ mode: 'edit', session: makeSession(), onClose: vi.fn() });
+
+    await screen.findByLabelText('Handeling');
+    expect(await screen.findByText(/00:50:00/)).toBeInTheDocument();
+  });
+
+  it('shows an inline error when the API returns 400', async () => {
+    const { ApiError } = await import('../lib/api');
+    mockApiFetch.mockImplementation((path: string) => {
+      if (path === '/api/admin/users') return Promise.resolve(USERS as never);
+      if (path === '/api/activities') return Promise.resolve(ACTIVITIES as never);
+      return Promise.reject(new ApiError(400, 'Invalid input'));
+    });
+
+    renderForm({ mode: 'edit', session: makeSession(), onClose: vi.fn() });
+
+    await screen.findByLabelText('Handeling');
+    await userEvent.click(screen.getByRole('button', { name: /Opslaan/ }));
+
+    expect(await screen.findByText(/Controleer de ingevoerde gegevens/)).toBeInTheDocument();
+  });
+});
diff --git a/apps/admin/src/components/SessionForm.tsx b/apps/admin/src/components/SessionForm.tsx
new file mode 100644
index 0000000..03792c4
--- /dev/null
+++ b/apps/admin/src/components/SessionForm.tsx
@@ -0,0 +1,279 @@
+import { useMemo, useState } from 'react';
+import type {
+  AdminUpdateSessionInput,
+  CreateManualSessionInput,
+  InsoleType,
+  SessionStatus,
+  WorkSession,
+} from '@solelog/shared';
+import { useActivities } from '../api/activities';
+import { useAdminUsers, useCreateManualSession, useUpdateSession } from '../api/admin-sessions';
+import { ApiError } from '../lib/api';
+import { formatTime } from '../lib/elapsed';
+
+const ALL_TYPES: InsoleType[] = ['Kurk', 'Berk', '3D'];
+
+const STATUS_OPTIONS: { value: SessionStatus; label: string }[] = [
+  { value: 'active', label: 'Actief' },
+  { value: 'completed', label: 'Voltooid' },
+  { value: 'discarded', label: 'Geannuleerd' },
+];
+
+// ISO-8601 → the `YYYY-MM-DDTHH:mm` shape a datetime-local input wants, in local time.
+function isoToLocal(iso: string | null): string {
+  if (!iso) return '';
+  const d = new Date(iso);
+  if (Number.isNaN(d.getTime())) return '';
+  const pad = (n: number) => String(n).padStart(2, '0');
+  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}`;
+}
+
+// A datetime-local value (local wall-clock) → an ISO-8601 instant.
+function localToIso(local: string): string | null {
+  if (!local) return null;
+  const d = new Date(local);
+  if (Number.isNaN(d.getTime())) return null;
+  return d.toISOString();
+}
+
+function spanSeconds(startLocal: string, endLocal: string): number | null {
+  const start = startLocal ? new Date(startLocal) : null;
+  const end = endLocal ? new Date(endLocal) : null;
+  if (!start || !end || Number.isNaN(start.getTime()) || Number.isNaN(end.getTime())) return null;
+  return Math.round((end.getTime() - start.getTime()) / 1000);
+}
+
+type Props = {
+  mode: 'create' | 'edit';
+  session?: WorkSession;
+  onClose: () => void;
+};
+
+export default function SessionForm({ mode, session, onClose }: Props) {
+  const usersQuery = useAdminUsers();
+  const activitiesQuery = useActivities();
+  const createSession = useCreateManualSession();
+  const updateSession = useUpdateSession();
+
+  const users = usersQuery.data ?? [];
+  const activities = activitiesQuery.data ?? [];
+
+  const [userId, setUserId] = useState(session?.user_id ?? '');
+  const [activityId, setActivityId] = useState<number | ''>(session?.activity_id ?? '');
+  const [insoleType, setInsoleType] = useState<InsoleType | ''>(session?.insole_type ?? '');
+  const [pairCount, setPairCount] = useState(session?.pair_count ?? 2);
+  const [startLocal, setStartLocal] = useState(isoToLocal(session?.start_time ?? null));
+  const [endLocal, setEndLocal] = useState(isoToLocal(session?.end_time ?? null));
+  const [pausedMinutes, setPausedMinutes] = useState(
+    session ? Math.round(session.paused_seconds / 60) : 0,
+  );
+  const [status, setStatus] = useState<SessionStatus>(session?.status ?? 'completed');
+  const [notes, setNotes] = useState(session?.notes ?? '');
+  const [error, setError] = useState<string | null>(null);
+
+  const pausedSeconds = Math.max(0, pausedMinutes) * 60;
+
+  const workedPreview = useMemo(() => {
+    const span = spanSeconds(startLocal, endLocal);
+    if (span === null) return null;
+    return Math.max(0, span - pausedSeconds);
+  }, [startLocal, endLocal, pausedSeconds]);
+
+  const submitting = createSession.isPending || updateSession.isPending;
+
+  function onError(err: unknown) {
+    if (err instanceof ApiError && err.status === 400) {
+      setError('Controleer de ingevoerde gegevens (eindtijd na starttijd, aantal ≥ 1).');
+    } else {
+      setError('Opslaan mislukt. Probeer het opnieuw.');
+    }
+  }
+
+  function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+    setError(null);
+
+    const startIso = localToIso(startLocal);
+    if (!startIso || activityId === '') {
+      setError('Vul handeling en starttijd in.');
+      return;
+    }
+
+    if (mode === 'create') {
+      const endIso = localToIso(endLocal);
+      if (!userId || !insoleType || !endIso) {
+        setError('Vul medewerker, zooltype, start- en eindtijd in.');
+        return;
+      }
+      const input: CreateManualSessionInput = {
+        user_id: userId,
+        activity_id: activityId,
+        insole_type: insoleType,
+        pair_count: pairCount,
+        start_time: startIso,
+        end_time: endIso,
+        paused_seconds: pausedSeconds,
+        notes: notes.trim() ? notes.trim() : null,
+      };
+      createSession.mutate(input, { onSuccess: () => onClose(), onError });
+      return;
+    }
+
+    if (!session) return;
+    const input: AdminUpdateSessionInput = {
+      activity_id: activityId,
+      insole_type: insoleType === '' ? null : insoleType,
+      pair_count: pairCount,
+      start_time: startIso,
+      end_time: status === 'active' ? null : localToIso(endLocal),
+      paused_seconds: pausedSeconds,
+      notes: notes.trim() ? notes.trim() : null,
+      status,
+    };
+    updateSession.mutate({ id: session.id, input }, { onSuccess: () => onClose(), onError });
+  }
+
+  return (
+    <form className="card session-form" onSubmit={handleSubmit}>
+      <h2 className="section-label">
+        {mode === 'create' ? 'Nieuwe registratie' : 'Sessie bewerken'}
+      </h2>
+
+      {mode === 'create' && (
+        <label className="session-field">
+          <span className="sub-label">Medewerker</span>
+          <select
+            aria-label="Medewerker"
+            value={userId}
+            onChange={(e) => setUserId(e.target.value)}
+          >
+            <option value="">— Kies —</option>
+            {users.map((u) => (
+              <option key={u.id} value={u.id}>
+                {u.name}
+              </option>
+            ))}
+          </select>
+        </label>
+      )}
+
+      <label className="session-field">
+        <span className="sub-label">Handeling</span>
+        <select
+          aria-label="Handeling"
+          value={activityId}
+          onChange={(e) => setActivityId(e.target.value === '' ? '' : Number(e.target.value))}
+        >
+          <option value="">— Kies —</option>
+          {activities.map((a) => (
+            <option key={a.id} value={a.id}>
+              {a.name}
+            </option>
+          ))}
+        </select>
+      </label>
+
+      <label className="session-field">
+        <span className="sub-label">Zooltype</span>
+        <select
+          aria-label="Zooltype"
+          value={insoleType}
+          onChange={(e) =>
+            setInsoleType(e.target.value === '' ? '' : (e.target.value as InsoleType))
+          }
+        >
+          <option value="">— Kies —</option>
+          {ALL_TYPES.map((t) => (
+            <option key={t} value={t}>
+              {t}
+            </option>
+          ))}
+        </select>
+      </label>
+
+      <label className="session-field">
+        <span className="sub-label">Aantal paren</span>
+        <input
+          type="number"
+          min={1}
+          aria-label="Aantal paren"
+          value={pairCount}
+          onChange={(e) => setPairCount(Math.max(1, Number(e.target.value) || 1))}
+        />
+      </label>
+
+      <label className="session-field">
+        <span className="sub-label">Starttijd</span>
+        <input
+          type="datetime-local"
+          aria-label="Starttijd"
+          value={startLocal}
+          onChange={(e) => setStartLocal(e.target.value)}
+        />
+      </label>
+
+      <label className="session-field">
+        <span className="sub-label">Eindtijd</span>
+        <input
+          type="datetime-local"
+          aria-label="Eindtijd"
+          value={endLocal}
+          onChange={(e) => setEndLocal(e.target.value)}
+        />
+      </label>
+
+      <label className="session-field">
+        <span className="sub-label">Pauze (minuten)</span>
+        <input
+          type="number"
+          min={0}
+          aria-label="Pauze (minuten)"
+          value={pausedMinutes}
+          onChange={(e) => setPausedMinutes(Math.max(0, Number(e.target.value) || 0))}
+        />
+      </label>
+
+      {mode === 'edit' && (
+        <label className="session-field">
+          <span className="sub-label">Status</span>
+          <select
+            aria-label="Status"
+            value={status}
+            onChange={(e) => setStatus(e.target.value as SessionStatus)}
+          >
+            {STATUS_OPTIONS.map((opt) => (
+              <option key={opt.value} value={opt.value}>
+                {opt.label}
+              </option>
+            ))}
+          </select>
+        </label>
+      )}
+
+      <label className="session-field">
+        <span className="sub-label">Notitie</span>
+        <textarea
+          aria-label="Notitie"
+          rows={2}
+          value={notes}
+          onChange={(e) => setNotes(e.target.value)}
+        />
+      </label>
+
+      <p className="session-preview">
+        Gewerkt: <strong>{workedPreview === null ? '—' : formatTime(workedPreview)}</strong>
+      </p>
+
+      {error && <p className="login-error">{error}</p>}
+
+      <div className="row-actions">
+        <button type="submit" className="btn-save" disabled={submitting}>
+          Opslaan
+        </button>
+        <button type="button" className="btn-cancel" onClick={onClose} disabled={submitting}>
+          Annuleren
+        </button>
+      </div>
+    </form>
+  );
+}
diff --git a/apps/admin/src/screens/Sessions.tsx b/apps/admin/src/screens/Sessions.tsx
index d249893..fb35b3f 100644
--- a/apps/admin/src/screens/Sessions.tsx
+++ b/apps/admin/src/screens/Sessions.tsx
@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import type { SessionStatus, WorkSession } from '@solelog/shared';
 import { useAdminDiscardSession, useAdminStopSession, useAllSessions } from '../api/admin-sessions';
+import SessionForm from '../components/SessionForm';
 import { formatTime } from '../lib/elapsed';
 
 type StatusFilter = 'all' | SessionStatus;
@@ -43,8 +44,10 @@ export default function Sessions() {
   const discardSession = useAdminDiscardSession();
 
   const [filter, setFilter] = useState<StatusFilter>('all');
-  // Task 4 replaces this stub with the real create/edit form.
-  const [formOpen, setFormOpen] = useState(false);
+  // null = closed; { mode } drives the shared create/edit form.
+  const [form, setForm] = useState<
+    { mode: 'create' } | { mode: 'edit'; session: WorkSession } | null
+  >(null);
 
   if (isError) {
     return (
@@ -62,7 +65,7 @@ export default function Sessions() {
     <div className="screen">
       <div className="sessions-head">
         <h1 className="screen-title">Sessies</h1>
-        <button type="button" className="btn-primary" onClick={() => setFormOpen(true)}>
+        <button type="button" className="btn-primary" onClick={() => setForm({ mode: 'create' })}>
           + Nieuwe registratie
         </button>
       </div>
@@ -84,20 +87,14 @@ export default function Sessions() {
         </label>
       </div>
 
-      {formOpen && (
-        <section className="card" data-testid="session-form">
-          <h2 className="section-label">Nieuwe registratie</h2>
-          {/* Task 4 fills in the full create/edit form. */}
-          <label className="sessions-filter">
-            Medewerker
-            <select aria-label="Medewerker" disabled>
-              <option>—</option>
-            </select>
-          </label>
-          <button type="button" className="btn-cancel" onClick={() => setFormOpen(false)}>
-            Annuleren
-          </button>
-        </section>
+      {form && (
+        <div data-testid="session-form">
+          <SessionForm
+            mode={form.mode}
+            session={form.mode === 'edit' ? form.session : undefined}
+            onClose={() => setForm(null)}
+          />
+        </div>
       )}
 
       {isLoading ? (
@@ -114,6 +111,7 @@ export default function Sessions() {
               session={session}
               onStop={() => stopSession.mutate(session.id)}
               onDiscard={() => discardSession.mutate(session.id)}
+              onEdit={() => setForm({ mode: 'edit', session })}
             />
           ))}
         </ul>
@@ -126,10 +124,12 @@ function SessionRow({
   session,
   onStop,
   onDiscard,
+  onEdit,
 }: {
   session: WorkSession;
   onStop: () => void;
   onDiscard: () => void;
+  onEdit: () => void;
 }) {
   const worked = workedSeconds(session);
   const isActive = session.status === 'active';
@@ -158,6 +158,7 @@ function SessionRow({
           type="button"
           className="icon-btn icon-edit"
           aria-label={`Bewerk sessie van ${session.user_name ?? 'onbekend'}`}
+          onClick={onEdit}
         >
           ✎
         </button>
diff --git a/apps/admin/src/styles.css b/apps/admin/src/styles.css
index d461bc9..c0c4ccc 100644
--- a/apps/admin/src/styles.css
+++ b/apps/admin/src/styles.css
@@ -577,3 +577,40 @@ body {
   border-radius: 12px;
   cursor: pointer;
 }
+
+/* ---- Session create/edit form ---- */
+.session-form {
+  gap: 14px;
+}
+
+.session-field {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.session-field select,
+.session-field input,
+.session-field textarea {
+  padding: 10px 12px;
+  font-size: 15px;
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  background: #ffffff;
+  color: var(--text);
+  font-family: inherit;
+}
+
+.session-field select:focus,
+.session-field input:focus,
+.session-field textarea:focus {
+  outline: none;
+  border-color: var(--primary);
+}
+
+.session-preview {
+  margin: 0;
+  font-size: 15px;
+  color: var(--text-muted);
+  font-variant-numeric: tabular-nums;
+}