diff --git a/src/lib/components/session-warning-modal.svelte b/src/lib/components/session-warning-modal.svelte
new file mode 100644
index 0000000000..815d9f242a
--- /dev/null
+++ b/src/lib/components/session-warning-modal.svelte
@@ -0,0 +1,115 @@
+<script lang="ts">
+  import { onDestroy } from 'svelte';
+
+  import Button from '$lib/holocene/button.svelte';
+  import { translate } from '$lib/i18n/translate';
+  import { logout } from '$lib/stores/auth-user';
+  import {
+    dismissSessionWarning,
+    sessionWarningState,
+  } from '$lib/stores/session-warning';
+  import { refreshTokens } from '$lib/utilities/auth-refresh';
+
+  const WARNING_SECONDS = 60;
+
+  let countdown = WARNING_SECONDS;
+  let extending = false;
+  let dialogEl: HTMLDialogElement;
+  let countdownInterval: ReturnType<typeof setInterval> | null = null;
+
+  $: isWarning = $sessionWarningState === 'warning';
+  $: isExpired = $sessionWarningState === 'expired';
+  $: open = isWarning || isExpired;
+
+  $: if (open && dialogEl) {
+    dialogEl.showModal();
+  } else if (!open && dialogEl) {
+    dialogEl.close();
+  }
+
+  $: if (isWarning) {
+    countdown = WARNING_SECONDS;
+    startCountdown();
+  } else {
+    stopCountdown();
+  }
+
+  function startCountdown() {
+    stopCountdown();
+    countdownInterval = setInterval(() => {
+      countdown -= 1;
+      if (countdown <= 0) {
+        stopCountdown();
+        sessionWarningState.set('expired');
+      }
+    }, 1000);
+  }
+
+  function stopCountdown() {
+    if (countdownInterval) {
+      clearInterval(countdownInterval);
+      countdownInterval = null;
+    }
+  }
+
+  async function handleExtend() {
+    extending = true;
+    const refreshed = await refreshTokens();
+    extending = false;
+    if (refreshed) {
+      dismissSessionWarning();
+    } else {
+      sessionWarningState.set('expired');
+    }
+  }
+
+  async function handleSignOut() {
+    await logout();
+  }
+
+  onDestroy(stopCountdown);
+</script>
+
+<dialog
+  bind:this={dialogEl}
+  role="alertdialog"
+  aria-modal="true"
+  aria-labelledby="session-warning-title"
+  aria-describedby="session-warning-body"
+  data-testid="session-warning-modal"
+  class="surface-primary z-50 w-full max-w-lg overflow-y-auto border border-secondary p-0 text-primary shadow-xl backdrop:bg-black/50"
+>
+  <div class="px-8 pb-0 pt-8 text-2xl">
+    <h2 id="session-warning-title">
+      {isExpired
+        ? translate('common.session-expired-title')
+        : translate('common.session-warning-title')}
+    </h2>
+  </div>
+  <div class="whitespace-normal p-8">
+    <p id="session-warning-body">
+      {isExpired
+        ? translate('common.session-expired-body')
+        : translate('common.session-warning-body', { seconds: countdown })}
+    </p>
+    {#if isWarning}
+      <p aria-live="polite" aria-atomic="true" class="sr-only">
+        {translate('common.session-warning-body', { seconds: countdown })}
+      </p>
+    {/if}
+  </div>
+  <div class="flex items-center justify-end gap-2 p-6">
+    {#if isExpired}
+      <Button variant="primary" on:click={handleSignOut}>
+        {translate('common.session-expired-sign-in-again')}
+      </Button>
+    {:else}
+      <Button variant="ghost" on:click={handleSignOut}>
+        {translate('common.session-warning-sign-out')}
+      </Button>
+      <Button variant="primary" loading={extending} on:click={handleExtend}>
+        {translate('common.session-warning-extend')}
+      </Button>
+    {/if}
+  </div>
+</dialog>
diff --git a/src/lib/i18n/locales/en/common.ts b/src/lib/i18n/locales/en/common.ts
index 6c89619dac..95968a0ea7 100644
--- a/src/lib/i18n/locales/en/common.ts
+++ b/src/lib/i18n/locales/en/common.ts
@@ -227,4 +227,13 @@ export const Strings = {
   'change-log': 'Change Log',
   comfortable: 'Comfortable',
   dense: 'Dense',
+  'session-warning-title': 'Your session is about to expire',
+  'session-warning-body':
+    'Your session will expire in {{seconds}} seconds. Stay signed in to keep working.',
+  'session-warning-extend': 'Stay signed in',
+  'session-warning-sign-out': 'Sign out',
+  'session-expired-title': 'Your session has expired',
+  'session-expired-body':
+    'Your session has expired. Sign in again to continue.',
+  'session-expired-sign-in-again': 'Sign in again',
 } as const;
diff --git a/src/lib/stores/auth-user.ts b/src/lib/stores/auth-user.ts
index a158ebb029..58087a94ef 100644
--- a/src/lib/stores/auth-user.ts
+++ b/src/lib/stores/auth-user.ts
@@ -1,12 +1,38 @@
 import { get } from 'svelte/store';
 
 import { persistStore } from '$lib/stores/persist-store';
+import {
+  dismissSessionWarning,
+  sessionWarningState,
+} from '$lib/stores/session-warning';
 import type { User } from '$lib/types/global';
 
 export const authUser = persistStore<User>('AuthUser', {});
 
 export const getAuthUser = (): User => get(authUser);
 
+const SESSION_WARNING_LEAD_SECS = 60;
+let warningTimer: ReturnType<typeof setTimeout> | null = null;
+
+function decodeJwtExp(token: string): number | null {
+  try {
+    const payload = JSON.parse(atob(token.split('.')[1]));
+    return typeof payload.exp === 'number' ? payload.exp : null;
+  } catch {
+    return null;
+  }
+}
+
+function scheduleSessionWarning(exp: number) {
+  if (warningTimer) clearTimeout(warningTimer);
+  const now = Math.floor(Date.now() / 1000);
+  const delay = (exp - now - SESSION_WARNING_LEAD_SECS) * 1000;
+  if (delay <= 0) return;
+  warningTimer = setTimeout(() => {
+    sessionWarningState.set('warning');
+  }, delay);
+}
+
 export const setAuthUser = (user: User) => {
   const { accessToken, idToken, name, email, picture } = user;
 
@@ -21,9 +47,17 @@ export const setAuthUser = (user: User) => {
     email,
     picture,
   });
+
+  dismissSessionWarning();
+  const exp = decodeJwtExp(accessToken);
+  if (exp) scheduleSessionWarning(exp);
 };
 
 export const clearAuthUser = () => {
+  if (warningTimer) {
+    clearTimeout(warningTimer);
+    warningTimer = null;
+  }
   authUser.set({});
 };
 
diff --git a/src/lib/stores/session-warning.ts b/src/lib/stores/session-warning.ts
new file mode 100644
index 0000000000..f96f743bb1
--- /dev/null
+++ b/src/lib/stores/session-warning.ts
@@ -0,0 +1,13 @@
+import { writable } from 'svelte/store';
+
+export type SessionWarningState = 'idle' | 'warning' | 'expired';
+
+export const sessionWarningState = writable<SessionWarningState>('idle');
+
+export const triggerSessionExpired = () => {
+  sessionWarningState.set('expired');
+};
+
+export const dismissSessionWarning = () => {
+  sessionWarningState.set('idle');
+};
diff --git a/src/lib/utilities/handle-error.test.ts b/src/lib/utilities/handle-error.test.ts
index 6b76890453..8fefbee14c 100644
--- a/src/lib/utilities/handle-error.test.ts
+++ b/src/lib/utilities/handle-error.test.ts
@@ -1,5 +1,13 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
+vi.mock('$lib/stores/session-warning', () => ({
+  triggerSessionExpired: vi.fn(),
+  dismissSessionWarning: vi.fn(),
+  sessionWarningState: { subscribe: vi.fn(), set: vi.fn() },
+}));
+
+import { triggerSessionExpired } from '$lib/stores/session-warning';
+
 import { handleError } from './handle-error';
 import { routeForLoginPage } from './route-for';
 
@@ -20,26 +28,28 @@ describe('handleError', () => {
     vi.clearAllMocks();
   });
 
-  it('should redirect if it is an unauthorized error with status', () => {
+  it('should trigger session expired if it is an unauthorized error with status', () => {
     const error = {
       status: 401,
       statusText: 'Unauthorized',
       response: null as unknown as Response,
     };
 
-    expect(() => handleError(error)).toThrowError();
-    expect(window.location.assign).toHaveBeenCalledWith(routeForLoginPage());
+    handleError(error);
+    expect(triggerSessionExpired).toHaveBeenCalled();
+    expect(window.location.assign).not.toHaveBeenCalled();
   });
 
-  it('should redirect if it is an unauthorized error with statusCode', () => {
+  it('should trigger session expired if it is an unauthorized error with statusCode', () => {
     const error = {
       statusCode: 401,
       statusText: 'Unauthorized',
       response: null as unknown as Response,
     };
 
-    expect(() => handleError(error)).toThrowError();
-    expect(window.location.assign).toHaveBeenCalledWith(routeForLoginPage());
+    handleError(error);
+    expect(triggerSessionExpired).toHaveBeenCalled();
+    expect(window.location.assign).not.toHaveBeenCalled();
   });
 
   it('should redirect if it is a forbidden error with status', () => {
diff --git a/src/lib/utilities/handle-error.ts b/src/lib/utilities/handle-error.ts
index 26a1c369f3..0285e2b209 100644
--- a/src/lib/utilities/handle-error.ts
+++ b/src/lib/utilities/handle-error.ts
@@ -1,6 +1,7 @@
 import { BROWSER } from 'esm-env';
 
 import { networkError } from '$lib/stores/error';
+import { triggerSessionExpired } from '$lib/stores/session-warning';
 import { toaster } from '$lib/stores/toaster';
 import type { NetworkError } from '$lib/types/global';
 
@@ -34,7 +35,8 @@ export const handleError = (
   }
 
   if (isUnauthorized(error) && isBrowser) {
-    window.location.assign(routeForLoginPage());
+    triggerSessionExpired();
+    return;
   }
 
   if (isForbidden(error) && isBrowser) {
@@ -58,7 +60,7 @@ export const handleUnauthorizedOrForbiddenError = (
   isBrowser = BROWSER,
 ): void => {
   if (isUnauthorized(error) && isBrowser) {
-    window.location.assign(routeForLoginPage());
+    triggerSessionExpired();
     return;
   }
 
diff --git a/src/routes/(app)/+layout.svelte b/src/routes/(app)/+layout.svelte
index 96f07b94c2..a66ffa4d64 100644
--- a/src/routes/(app)/+layout.svelte
+++ b/src/routes/(app)/+layout.svelte
@@ -7,6 +7,7 @@
   import BottomNavigation from '$lib/components/bottom-nav.svelte';
   import DataEncoderSettings from '$lib/components/data-encoder-settings.svelte';
   import NamespacePicker from '$lib/components/namespace-picker.svelte';
+  import SessionWarningModal from '$lib/components/session-warning-modal.svelte';
   import SideNavigation from '$lib/components/side-nav.svelte';
   import SkipNavigation from '$lib/components/skip-nav.svelte';
   import TopNavigation from '$lib/components/top-nav.svelte';
@@ -304,6 +305,7 @@
 
 <DarkMode />
 <SkipNavigation />
+<SessionWarningModal />
 
 <div class="flex h-dvh w-screen flex-row">
   <Toaster