diff --git a/e2e/auth/session-expiry.spec.ts b/e2e/auth/session-expiry.spec.ts
new file mode 100644
index 00000000..6fdd8c65
--- /dev/null
+++ b/e2e/auth/session-expiry.spec.ts
@@ -0,0 +1,90 @@
+import { test, expect, type Page } from '@playwright/test';
+import { waitForHydration } from '../support/helpers.js';
+
+/** Issue a same-origin fetch from the page and wait for the session check it triggers. */
+async function fetchAndAwaitSessionCheck(page: Page, url: string) {
+  const sessionCheck = page.waitForResponse((res) => res.url().includes('/api/auth/get-session'));
+  const status = await page.evaluate(async (u) => (await fetch(u)).status, url);
+  await sessionCheck;
+  return status;
+}
+
+test.describe('Session expiry', () => {
+  test.use({ locale: 'en-US' });
+
+  test.describe('without a session', () => {
+    test.use({ storageState: { cookies: [], origins: [] } });
+
+    test('API routes return a JSON 401', async ({ request }) => {
+      const res = await request.get('/api/trino/catalog?level=catalogs', { maxRedirects: 0 });
+      expect(res.status()).toBe(401);
+      expect(await res.json()).toEqual({ message: 'Authentication required' });
+    });
+  });
+
+  test('shows a blocking modal when an API call fails after the session is gone', async ({
+    page
+  }) => {
+    await page.goto('/trino?tab=1');
+    await waitForHydration(page);
+
+    const dialog = page.getByRole('dialog', { name: 'Session expired' });
+    await expect(dialog).toBeHidden();
+
+    await page.context().clearCookies();
+    const status = await fetchAndAwaitSessionCheck(page, '/api/trino/catalog?level=catalogs');
+    expect(status).toBe(401);
+
+    await expect(dialog).toBeVisible();
+    await expect(dialog.getByText('Your session has expired.')).toBeVisible();
+
+    const signIn = dialog.getByRole('link', { name: 'Sign in again' });
+
+    // The modal cannot be dismissed. Press Escape twice: browsers may skip the
+    // cancellable `cancel` event on a repeated Escape and close the dialog.
+    await page.keyboard.press('Escape');
+    await page.keyboard.press('Escape');
+    await expect(dialog).toBeVisible();
+    await expect(signIn).toBeFocused();
+
+    await expect(signIn).toHaveAttribute(
+      'href',
+      '/auth/login?redirectTo=' + encodeURIComponent('/trino?tab=1')
+    );
+
+    // Signing in again returns the user to where they were. This relies on the
+    // mock OIDC provider signing in without a login form.
+    await signIn.click();
+    await expect(page).toHaveURL(/\/auth\/login/);
+    await waitForHydration(page);
+    await page.getByRole('button', { name: /sign in with sso/i }).click();
+    await expect(page).toHaveURL('/trino?tab=1');
+    await expect(dialog).toBeHidden();
+  });
+
+  test('shows the modal when the user returns to the tab after the session is gone', async ({
+    page
+  }) => {
+    await page.goto('/');
+    await waitForHydration(page);
+
+    await page.context().clearCookies();
+    const sessionCheck = page.waitForResponse((res) => res.url().includes('/api/auth/get-session'));
+    await page.evaluate(() => document.dispatchEvent(new Event('visibilitychange')));
+    await sessionCheck;
+
+    await expect(page.getByRole('dialog', { name: 'Session expired' })).toBeVisible();
+  });
+
+  test('does not show the modal for a 401 while the session is valid', async ({ page }) => {
+    await page.goto('/');
+    await waitForHydration(page);
+
+    // The storage API answers 401 when no storage connection header is sent
+    // (requires the storage browser to be enabled, as in .env.test).
+    const status = await fetchAndAwaitSessionCheck(page, '/api/storage/buckets');
+    expect(status).toBe(401);
+
+    await expect(page.getByRole('dialog', { name: 'Session expired' })).toBeHidden();
+  });
+});
diff --git a/messages/de.json b/messages/de.json
index 8431a757..54b1d0ee 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -44,6 +44,9 @@
   "auth_login_subtitle": "Verwenden Sie den SSO-Anbieter Ihrer Organisation, um auf die Plattform zuzugreifen.",
   "auth_login_button": "Mit SSO anmelden",
   "auth_login_error": "Anmeldung fehlgeschlagen. Bitte versuchen Sie es erneut.",
+  "session_expired_title": "Sitzung abgelaufen",
+  "session_expired_message": "Ihre Sitzung ist abgelaufen. Melden Sie sich erneut an, um fortzufahren.",
+  "session_expired_sign_in": "Erneut anmelden",
   "header_sign_out": "Abmelden",
   "trino_results_empty": "Keine Ergebnisse",
   "trino_query_error": "Abfragefehler",
diff --git a/messages/en.json b/messages/en.json
index 92a586a0..3cd59ee7 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -44,6 +44,9 @@
   "auth_login_subtitle": "Use your organisation's SSO provider to access the platform.",
   "auth_login_button": "Sign in with SSO",
   "auth_login_error": "Sign-in failed. Please try again.",
+  "session_expired_title": "Session expired",
+  "session_expired_message": "Your session has expired. Sign in again to continue.",
+  "session_expired_sign_in": "Sign in again",
   "header_sign_out": "Sign out",
   "trino_results_empty": "No results",
   "trino_query_error": "Query error",
diff --git a/src/hooks.client.ts b/src/hooks.client.ts
new file mode 100644
index 00000000..4de7dab2
--- /dev/null
+++ b/src/hooks.client.ts
@@ -0,0 +1,28 @@
+import type { ClientInit } from '@sveltejs/kit';
+import { checkSession } from '$lib/client/session.svelte';
+
+/**
+ * Wrap `window.fetch` so that any same-origin 401 triggers a session check.
+ * A 401 does not always mean the session has expired (e.g. the storage API
+ * uses it for a missing connection), so the response itself is passed through
+ * unchanged and `checkSession` decides whether the session is really gone.
+ */
+export const init: ClientInit = () => {
+  const originalFetch = window.fetch;
+  window.fetch = async (input, requestInit) => {
+    const response = await originalFetch(input, requestInit);
+    if (response.status === 401 && isSameOrigin(response.url)) {
+      void checkSession();
+    }
+    return response;
+  };
+};
+
+function isSameOrigin(url: string): boolean {
+  if (!url) return false;
+  try {
+    return new URL(url).origin === window.location.origin;
+  } catch {
+    return false;
+  }
+}
diff --git a/src/hooks.server.ts b/src/hooks.server.ts
index 0cd63943..a6cc19e6 100644
--- a/src/hooks.server.ts
+++ b/src/hooks.server.ts
@@ -1,10 +1,11 @@
 import { paraglideMiddleware } from '$lib/paraglide/server';
 import { httpRequestDuration } from '$lib/server/metrics';
 import { building, dev } from '$app/environment';
-import { error, redirect, type Handle, type HandleServerError } from '@sveltejs/kit';
+import { error, type Handle, type HandleServerError } from '@sveltejs/kit';
 import { sequence } from '@sveltejs/kit/hooks';
 import { svelteKitHandler } from 'better-auth/svelte-kit';
 import { auth, oidcEnabled } from '$lib/server/auth';
+import { unauthenticatedResponse } from '$lib/server/auth-guard';
 import { requestLogger, logger } from '$lib/server/logging';
 import { getConnectionFromHeader } from '$lib/server/storage/connection.js';
 import { storageBrowserEnabled } from '$lib/server/feature-flags.js';
@@ -53,8 +54,8 @@ const handleAuthGuard: Handle = async ({ event, resolve }) => {
 
   const isPublic = PUBLIC_PATHS.some((p) => event.url.pathname.startsWith(p));
   if (!isPublic && !event.locals.user) {
-    const redirectTo = encodeURIComponent(event.url.pathname + event.url.search);
-    throw redirect(302, `/auth/login?redirectTo=${redirectTo}`);
+    event.locals.logger.debug({ path: event.url.pathname }, 'Rejecting unauthenticated request');
+    return unauthenticatedResponse(event.url);
   }
 
   return resolve(event);
diff --git a/src/lib/client/session.svelte.spec.ts b/src/lib/client/session.svelte.spec.ts
new file mode 100644
index 00000000..3f894f4a
--- /dev/null
+++ b/src/lib/client/session.svelte.spec.ts
@@ -0,0 +1,66 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { checkSession, sessionState } from './session.svelte.js';
+
+const { getSession } = vi.hoisted(() => ({ getSession: vi.fn() }));
+vi.mock('$lib/auth-client', () => ({ authClient: { getSession } }));
+
+describe('checkSession', () => {
+  beforeEach(() => {
+    getSession.mockReset();
+    sessionState.active = true;
+    sessionState.expired = false;
+  });
+
+  it('marks the session expired when the server reports no session', async () => {
+    getSession.mockResolvedValue({ data: null, error: null });
+
+    await checkSession();
+
+    expect(sessionState.expired).toBe(true);
+    expect(getSession).toHaveBeenCalledWith({ query: { disableRefresh: true } });
+  });
+
+  it('keeps the session when the server returns one', async () => {
+    getSession.mockResolvedValue({ data: { user: {}, session: {} }, error: null });
+
+    await checkSession();
+
+    expect(sessionState.expired).toBe(false);
+  });
+
+  it('clears the flag once the session is restored', async () => {
+    sessionState.expired = true;
+    getSession.mockResolvedValue({ data: { user: {}, session: {} }, error: null });
+
+    await checkSession();
+
+    expect(sessionState.expired).toBe(false);
+  });
+
+  it('ignores server and network errors', async () => {
+    getSession.mockResolvedValueOnce({ data: null, error: { status: 500 } });
+    getSession.mockRejectedValueOnce(new TypeError('Failed to fetch'));
+
+    await checkSession();
+    await checkSession();
+
+    expect(sessionState.expired).toBe(false);
+  });
+
+  it('shares one request between concurrent checks', async () => {
+    getSession.mockResolvedValue({ data: { user: {}, session: {} }, error: null });
+
+    await Promise.all([checkSession(), checkSession(), checkSession()]);
+
+    expect(getSession).toHaveBeenCalledTimes(1);
+  });
+
+  it('does nothing while inactive', async () => {
+    sessionState.active = false;
+
+    await checkSession();
+
+    expect(getSession).not.toHaveBeenCalled();
+    expect(sessionState.expired).toBe(false);
+  });
+});
diff --git a/src/lib/client/session.svelte.ts b/src/lib/client/session.svelte.ts
new file mode 100644
index 00000000..02b21baf
--- /dev/null
+++ b/src/lib/client/session.svelte.ts
@@ -0,0 +1,36 @@
+import { authClient } from '$lib/auth-client';
+
+/**
+ * Client-side view of the user's session. `active` is set while a signed-in
+ * layout is mounted; without it (e.g. OIDC disabled) no checks are made.
+ */
+export const sessionState = $state({ active: false, expired: false });
+
+let pending: Promise<void> | null = null;
+
+/**
+ * Ask the server whether the current session is still valid and update
+ * `sessionState.expired`. Concurrent calls share one request. A session that
+ * is restored (e.g. by signing in again in another tab) clears the flag.
+ *
+ * Network and server errors are ignored: only an explicit "no session" answer
+ * marks the session as expired.
+ *
+ * Note: `disableRefresh` only prevents extending a database-backed session.
+ * In stateless mode (no database, as configured today) a check made shortly
+ * before the session cookie expires still renews it, like any other request.
+ */
+export function checkSession(): Promise<void> {
+  if (!sessionState.active) return Promise.resolve();
+  pending ??= authClient
+    .getSession({ query: { disableRefresh: true } })
+    .then(({ data, error }) => {
+      // Drop the result if the signed-in layout unmounted in the meantime.
+      if (!error && sessionState.active) sessionState.expired = !data;
+    })
+    .catch(() => {})
+    .finally(() => {
+      pending = null;
+    });
+  return pending;
+}
diff --git a/src/lib/components/Modal.svelte b/src/lib/components/Modal.svelte
index 99558c69..6130a725 100644
--- a/src/lib/components/Modal.svelte
+++ b/src/lib/components/Modal.svelte
@@ -1,12 +1,21 @@
 <script lang="ts">
+  import type { HTMLDialogAttributes } from 'svelte/elements';
+
   let {
     open = $bindable(false),
     children,
-    class: className = ''
-  }: {
+    class: className = '',
+    dismissible = true,
+    ...rest
+  }: Omit<
+    HTMLDialogAttributes,
+    'open' | 'class' | 'children' | 'onclose' | 'oncancel' | 'onclick'
+  > & {
     open: boolean;
     children: import('svelte').Snippet;
     class?: string;
+    /** When false, Escape and backdrop clicks do not close the modal. */
+    dismissible?: boolean;
   } = $props();
 
   let dialogEl = $state<HTMLDialogElement | undefined>(undefined);
@@ -22,19 +31,36 @@
   });
 
   function handleClose() {
+    // Browsers may close a dialog despite a cancelled `cancel` event (e.g. a
+    // repeated Escape press), so reopen it if it must stay open.
+    if (!dismissible && open) {
+      dialogEl?.showModal();
+      return;
+    }
     open = false;
   }
 
+  function handleCancel(e: Event) {
+    if (!dismissible) e.preventDefault();
+  }
+
   function handleBackdropClick(e: MouseEvent) {
     // The click target is the <dialog> element itself only when the backdrop is
     // clicked; clicks inside the content bubble up to child elements instead.
-    if (e.target === dialogEl) {
+    if (dismissible && e.target === dialogEl) {
       open = false;
     }
   }
 </script>
 
-<dialog bind:this={dialogEl} class={className} onclose={handleClose} onclick={handleBackdropClick}>
+<dialog
+  {...rest}
+  bind:this={dialogEl}
+  class={className}
+  onclose={handleClose}
+  oncancel={handleCancel}
+  onclick={handleBackdropClick}
+>
   {#if open}
     {@render children()}
   {/if}
diff --git a/src/lib/components/layout/SessionExpiredModal.svelte b/src/lib/components/layout/SessionExpiredModal.svelte
new file mode 100644
index 00000000..1eec6651
--- /dev/null
+++ b/src/lib/components/layout/SessionExpiredModal.svelte
@@ -0,0 +1,54 @@
+<script lang="ts">
+  import { page } from '$app/state';
+  import { resolve } from '$app/paths';
+  import IconLogin from 'virtual:icons/material-symbols/login';
+  import * as m from '$lib/paraglide/messages.js';
+  import Modal from '$lib/components/Modal.svelte';
+  import { checkSession, sessionState } from '$lib/client/session.svelte';
+
+  const uid = $props.id();
+
+  let redirectTo = $derived(encodeURIComponent(page.url.pathname + page.url.search));
+
+  // Enable session checks while mounted, and check when the user returns to
+  // the tab so an expired session is noticed before the next API call. There
+  // is deliberately no periodic check: in stateless mode a check can renew
+  // the session, so polling would keep an idle tab signed in.
+  $effect(() => {
+    sessionState.active = true;
+    const checkIfVisible = () => {
+      if (document.visibilityState === 'visible') void checkSession();
+    };
+    document.addEventListener('visibilitychange', checkIfVisible);
+    return () => {
+      sessionState.active = false;
+      document.removeEventListener('visibilitychange', checkIfVisible);
+    };
+  });
+</script>
+
+<Modal
+  bind:open={sessionState.expired}
+  dismissible={false}
+  class="modal"
+  aria-labelledby="{uid}-title"
+  aria-describedby="{uid}-message"
+>
+  <div class="modal-box max-w-md">
+    <h2 id="{uid}-title" class="text-lg font-bold">{m.session_expired_title()}</h2>
+    <p id="{uid}-message" class="text-base-content/70 py-4">{m.session_expired_message()}</p>
+    <div class="modal-action">
+      <!-- The path is resolved; the rule does not recognise the appended query string. -->
+      <!-- eslint-disable svelte/no-navigation-without-resolve -->
+      <a
+        href={`${resolve('/auth/login')}?redirectTo=${redirectTo}`}
+        class="btn btn-primary"
+        data-sveltekit-reload
+      >
+        <!-- eslint-enable svelte/no-navigation-without-resolve -->
+        <IconLogin class="size-5" aria-hidden="true" />
+        {m.session_expired_sign_in()}
+      </a>
+    </div>
+  </div>
+</Modal>
diff --git a/src/lib/server/auth-guard.test.ts b/src/lib/server/auth-guard.test.ts
new file mode 100644
index 00000000..4052be07
--- /dev/null
+++ b/src/lib/server/auth-guard.test.ts
@@ -0,0 +1,31 @@
+import { describe, expect, it } from 'vitest';
+import { isRedirect } from '@sveltejs/kit';
+import { unauthenticatedResponse } from './auth-guard.js';
+
+describe('unauthenticatedResponse', () => {
+  it('returns a JSON 401 for API routes', async () => {
+    const res = unauthenticatedResponse(
+      new URL('http://localhost/api/trino/catalog?level=catalogs')
+    );
+
+    expect(res.status).toBe(401);
+    expect(res.headers.get('content-type')).toContain('application/json');
+    expect(await res.json()).toEqual({ message: 'Authentication required' });
+  });
+
+  it('redirects page routes to the login page, preserving the target', () => {
+    try {
+      unauthenticatedResponse(new URL('http://localhost/trino?tab=1'));
+      expect.unreachable('expected a redirect to be thrown');
+    } catch (err) {
+      expect(isRedirect(err)).toBe(true);
+      if (!isRedirect(err)) return;
+      expect(err.status).toBe(302);
+      expect(err.location).toBe('/auth/login?redirectTo=%2Ftrino%3Ftab%3D1');
+    }
+  });
+
+  it('does not treat paths that merely start with "api" as API routes', () => {
+    expect(() => unauthenticatedResponse(new URL('http://localhost/apis'))).toThrow();
+  });
+});
diff --git a/src/lib/server/auth-guard.ts b/src/lib/server/auth-guard.ts
new file mode 100644
index 00000000..70d679e5
--- /dev/null
+++ b/src/lib/server/auth-guard.ts
@@ -0,0 +1,18 @@
+import { json, redirect } from '@sveltejs/kit';
+
+/**
+ * Build the response for a request that requires authentication but has no
+ * valid session.
+ *
+ * API routes get a JSON 401 so that client-side `fetch` callers receive a
+ * parseable error.
+ * The client treats a 401 as a hint to re-check the session (see
+ * `src/hooks.client.ts`). All other routes redirect to the login page.
+ */
+export function unauthenticatedResponse(url: URL): Response {
+  if (url.pathname.startsWith('/api/')) {
+    return json({ message: 'Authentication required' }, { status: 401 });
+  }
+  const redirectTo = encodeURIComponent(url.pathname + url.search);
+  throw redirect(302, `/auth/login?redirectTo=${redirectTo}`);
+}
diff --git a/src/routes/(app)/+layout.svelte b/src/routes/(app)/+layout.svelte
index f538f0f6..944903b7 100644
--- a/src/routes/(app)/+layout.svelte
+++ b/src/routes/(app)/+layout.svelte
@@ -5,6 +5,7 @@
   import Sidebar from '$lib/components/layout/sidebar/Sidebar.svelte';
   import Header from '$lib/components/layout/header/Header.svelte';
   import ToastHost from '$lib/components/ToastHost.svelte';
+  import SessionExpiredModal from '$lib/components/layout/SessionExpiredModal.svelte';
 
   let { children, data } = $props();
 
@@ -64,3 +65,8 @@
 </div>
 
 <ToastHost />
+
+<!-- Only signed-in users can lose their session; without OIDC there is none. -->
+{#if data.user}
+  <SessionExpiredModal />
+{/if}