diff --git a/web-client/src/AuthenticatedApp.tsx b/web-client/src/AuthenticatedApp.tsx
index e3d27b7..57cb1b3 100644
--- a/web-client/src/AuthenticatedApp.tsx
+++ b/web-client/src/AuthenticatedApp.tsx
@@ -1,10 +1,9 @@
 import { useEffect, useRef, useState } from 'react'
-import { AlertCircle } from 'lucide-react'
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { GlobalErrorBoundary } from '@/app/ErrorBoundary'
 import App from '@/App'
-import { Button } from '@/components/ui/button'
-import { Card, CardContent, CardDescription, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
+import { ErrorCard } from '@/components/ui/ErrorCard'
 import { LoadingSpinner } from '@/components/ui/LoadingSpinner'
-import { Alert, AlertDescription, AlertTitle } from '@/components/ui/alert'
 import keycloak, { KEYCLOAK_URL, TOKEN_REFRESH_MIN_VALIDITY_SECONDS } from '@/lib/keycloak'
 import {
   AUTH_INIT_TIMEOUT_MS,
@@ -17,7 +16,11 @@ function removeSplash() {
   document.getElementById('splash')?.remove()
 }
 
-export default function AuthenticatedApp() {
+interface AuthenticatedAppProps {
+  queryClient: QueryClient
+}
+
+export default function AuthenticatedApp({ queryClient }: AuthenticatedAppProps) {
   const [status, setStatus] = useState<'loading' | 'ready' | 'error'>('loading')
   const [authError, setAuthError] = useState<AuthError | null>(null)
   const didInitRef = useRef(false)
@@ -107,31 +110,14 @@ export default function AuthenticatedApp() {
     }
     const { title, description } = messages[authError ?? 'unknown']
 
-    return (
-      <div className="flex min-h-screen items-center justify-center bg-background">
-        <Card className="w-full max-w-sm">
-          <CardHeader>
-            <CardTitle>{title}</CardTitle>
-            <CardDescription>{description}</CardDescription>
-          </CardHeader>
-          <CardContent>
-            <Alert variant="destructive">
-              <AlertCircle className="size-4" />
-              <AlertTitle>Sign-in error</AlertTitle>
-              <AlertDescription>
-                If this keeps happening, contact support.
-              </AlertDescription>
-            </Alert>
-          </CardContent>
-          <CardFooter>
-            <Button className="w-full" onClick={() => window.location.reload()}>
-              Try again
-            </Button>
-          </CardFooter>
-        </Card>
-      </div>
-    )
+    return <ErrorCard title={title} description={description} alertTitle="Sign-in error" />
   }
 
-  return <App />
+  return (
+    <QueryClientProvider client={queryClient}>
+      <GlobalErrorBoundary>
+        <App />
+      </GlobalErrorBoundary>
+    </QueryClientProvider>
+  )
 }
diff --git a/web-client/src/__tests__/AuthenticatedApp.test.tsx b/web-client/src/__tests__/AuthenticatedApp.test.tsx
index 7067530..b2ee127 100644
--- a/web-client/src/__tests__/AuthenticatedApp.test.tsx
+++ b/web-client/src/__tests__/AuthenticatedApp.test.tsx
@@ -1,5 +1,6 @@
 import { StrictMode, act } from 'react'
 import { createRoot, type Root } from 'react-dom/client'
+import { QueryClient } from '@tanstack/react-query'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 
 const keycloakMock = {
@@ -16,6 +17,7 @@ vi.mock('@/App', () => ({
 vi.mock('@/lib/keycloak', () => ({
   KEYCLOAK_URL: 'http://keycloak.test',
   TOKEN_REFRESH_MIN_VALIDITY_SECONDS: 30,
+  createApiClient: vi.fn(() => ({ get: vi.fn(), post: vi.fn(), put: vi.fn(), delete: vi.fn() })),
   default: keycloakMock,
 }))
 
@@ -24,12 +26,14 @@ const { default: AuthenticatedApp } = await import('@/AuthenticatedApp')
 describe('AuthenticatedApp', () => {
   let container: HTMLDivElement
   let root: Root
+  let queryClient: QueryClient
 
   beforeEach(() => {
     vi.clearAllMocks()
     keycloakMock.onTokenExpired = undefined
     keycloakMock.login.mockResolvedValue(undefined)
     keycloakMock.updateToken.mockResolvedValue(true)
+    queryClient = new QueryClient()
     document.body.innerHTML = '<div id="root"></div><div id="splash"></div>'
     container = document.getElementById('root') as HTMLDivElement
     root = createRoot(container)
@@ -44,7 +48,7 @@ describe('AuthenticatedApp', () => {
 
   async function render() {
     await act(async () => {
-      root.render(<AuthenticatedApp />)
+      root.render(<AuthenticatedApp queryClient={queryClient} />)
       // flush microtasks so the async init chain resolves
       await new Promise((r) => setTimeout(r, 0))
     })
@@ -54,7 +58,7 @@ describe('AuthenticatedApp', () => {
     await act(async () => {
       root.render(
         <StrictMode>
-          <AuthenticatedApp />
+          <AuthenticatedApp queryClient={queryClient} />
         </StrictMode>,
       )
       await new Promise((r) => setTimeout(r, 0))
diff --git a/web-client/src/__tests__/ErrorBoundary.test.tsx b/web-client/src/__tests__/ErrorBoundary.test.tsx
new file mode 100644
index 0000000..8cdd63f
--- /dev/null
+++ b/web-client/src/__tests__/ErrorBoundary.test.tsx
@@ -0,0 +1,127 @@
+import { Component, useState, type ReactNode, act } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { GlobalErrorBoundary } from '@/app/ErrorBoundary'
+
+type ThrowingChildProps = {
+  shouldThrow: boolean
+}
+
+function ThrowingChild({ shouldThrow }: ThrowingChildProps) {
+  if (shouldThrow) {
+    throw new Error('Boom')
+  }
+
+  return <div>Child rendered</div>
+}
+
+let remountCount = 0
+
+function RemountSensitiveHarness() {
+  const [mountId] = useState(() => {
+    remountCount += 1
+    return remountCount
+  })
+
+  return <ThrowOnFirstMount mountId={mountId} />
+}
+
+type ThrowOnFirstMountProps = {
+  mountId: number
+}
+
+class ThrowOnFirstMount extends Component<ThrowOnFirstMountProps> {
+  componentDidMount() {
+    if (this.props.mountId === 1) {
+      throw new Error('Boom')
+    }
+  }
+
+  render() {
+    return <div>Child rendered</div>
+  }
+}
+
+describe('GlobalErrorBoundary', () => {
+  let container: HTMLDivElement
+  let root: Root
+  let consoleErrorSpy: ReturnType<typeof vi.spyOn>
+
+  beforeEach(() => {
+    document.body.innerHTML = '<div id="root"></div>'
+    container = document.getElementById('root') as HTMLDivElement
+    root = createRoot(container)
+    consoleErrorSpy = vi.spyOn(console, 'error').mockImplementation(() => {})
+    remountCount = 0
+  })
+
+  afterEach(async () => {
+    await act(async () => {
+      root.unmount()
+    })
+    consoleErrorSpy.mockRestore()
+    document.body.innerHTML = ''
+  })
+
+  async function render(node: ReactNode) {
+    await act(async () => {
+      root.render(node)
+    })
+  }
+
+  async function click(element: Element) {
+    await act(async () => {
+      element.dispatchEvent(new MouseEvent('click', { bubbles: true }))
+    })
+  }
+
+  it('renders children when no error is thrown', async () => {
+    await render(
+      <GlobalErrorBoundary>
+        <ThrowingChild shouldThrow={false} />
+      </GlobalErrorBoundary>,
+    )
+
+    expect(container.textContent).toContain('Child rendered')
+  })
+
+  it('renders the fallback when a child throws', async () => {
+    await render(
+      <GlobalErrorBoundary>
+        <ThrowingChild shouldThrow />
+      </GlobalErrorBoundary>,
+    )
+
+    expect(container.textContent).toContain('Go home')
+    expect(container.textContent).toContain('Try again')
+  })
+
+  it('does not render children when in error state', async () => {
+    await render(
+      <GlobalErrorBoundary>
+        <ThrowingChild shouldThrow />
+      </GlobalErrorBoundary>,
+    )
+
+    expect(container.textContent).not.toContain('Child rendered')
+  })
+
+  it('clicking "Try again" remounts the child subtree', async () => {
+    await render(
+      <GlobalErrorBoundary>
+        <RemountSensitiveHarness />
+      </GlobalErrorBoundary>,
+    )
+
+    const tryAgainButton = Array.from(container.querySelectorAll('button')).find(
+      (button) => button.textContent === 'Try again',
+    )
+    expect(tryAgainButton).toBeTruthy()
+
+    await click(tryAgainButton as Element)
+
+    expect(container.textContent).toContain('Child rendered')
+    expect(container.textContent).not.toContain('Try again')
+    expect(remountCount).toBe(2)
+  })
+})
diff --git a/web-client/src/__tests__/NotFoundPage.test.tsx b/web-client/src/__tests__/NotFoundPage.test.tsx
new file mode 100644
index 0000000..7b6f1c8
--- /dev/null
+++ b/web-client/src/__tests__/NotFoundPage.test.tsx
@@ -0,0 +1,61 @@
+import { act } from 'react'
+import { createRoot, type Root } from 'react-dom/client'
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { NotFoundPage } from '@/app/pages/NotFoundPage'
+
+describe('NotFoundPage', () => {
+  let container: HTMLDivElement
+  let root: Root
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+    document.body.innerHTML = '<div id="root"></div>'
+    container = document.getElementById('root') as HTMLDivElement
+    root = createRoot(container)
+  })
+
+  afterEach(async () => {
+    await act(async () => {
+      root.unmount()
+    })
+    document.body.innerHTML = ''
+  })
+
+  async function render() {
+    await act(async () => {
+      root.render(<NotFoundPage />)
+    })
+  }
+
+  it('renders the "404 - Page not found" text in the heading', async () => {
+    await render()
+
+    const heading = container.querySelector('h1')
+
+    expect(heading?.textContent).toContain('404 - Page not found')
+  })
+
+  it('renders a Go back button', async () => {
+    await render()
+
+    const button = container.querySelector('button')
+
+    expect(button?.textContent).toBe('Go back')
+  })
+
+  it('clicking Go back calls window.history.back()', async () => {
+    const backSpy = vi.spyOn(window.history, 'back').mockImplementation(() => {})
+
+    await render()
+
+    const button = container.querySelector('button')
+
+    expect(button).not.toBeNull()
+
+    await act(async () => {
+      button?.dispatchEvent(new MouseEvent('click', { bubbles: true }))
+    })
+
+    expect(backSpy).toHaveBeenCalledTimes(1)
+  })
+})
diff --git a/web-client/src/app/ErrorBoundary.tsx b/web-client/src/app/ErrorBoundary.tsx
new file mode 100644
index 0000000..e44d0da
--- /dev/null
+++ b/web-client/src/app/ErrorBoundary.tsx
@@ -0,0 +1,43 @@
+import { Component, Fragment, type ErrorInfo, type PropsWithChildren } from 'react'
+import { router } from '@/app/router/routes'
+import { ErrorCard } from '@/components/ui/ErrorCard'
+
+type GlobalErrorBoundaryState = {
+  hasError: boolean
+  resetKey: number
+}
+
+export class GlobalErrorBoundary extends Component<PropsWithChildren, GlobalErrorBoundaryState> {
+  state: GlobalErrorBoundaryState = {
+    hasError: false,
+    resetKey: 0,
+  }
+
+  static getDerivedStateFromError(): Pick<GlobalErrorBoundaryState, 'hasError'> {
+    return { hasError: true }
+  }
+
+  componentDidCatch(error: Error, errorInfo: ErrorInfo) {
+    console.error('GlobalErrorBoundary caught an error', error, errorInfo)
+  }
+
+  render() {
+    if (!this.state.hasError) {
+      return <Fragment key={this.state.resetKey}>{this.props.children}</Fragment>
+    }
+
+    return (
+      <ErrorCard
+        title="Something went wrong"
+        description="An unexpected error occurred. Please try again."
+        actions={[
+          { label: 'Go home', onClick: () => void router.navigate('/') },
+          {
+            label: 'Try again',
+            onClick: () => this.setState((s) => ({ hasError: false, resetKey: s.resetKey + 1 })),
+          },
+        ]}
+      />
+    )
+  }
+}
diff --git a/web-client/src/app/pages/NotFoundPage.tsx b/web-client/src/app/pages/NotFoundPage.tsx
new file mode 100644
index 0000000..d875ce4
--- /dev/null
+++ b/web-client/src/app/pages/NotFoundPage.tsx
@@ -0,0 +1,23 @@
+import { router } from '@/app/router/routes'
+import { Button } from '@/components/ui/button'
+
+export function NotFoundPage() {
+  return (
+    <div className="flex min-h-[calc(100dvh-3rem)] flex-col items-center justify-center gap-4 text-center">
+      <div className="space-y-2">
+        <h1 className="font-display text-display-md uppercase tracking-wide text-balance text-foreground">
+          404 - Page not found
+        </h1>
+        <p className="text-body-sm text-muted-foreground sm:text-body">
+          The page you&apos;re looking for doesn&apos;t exist.
+        </p>
+      </div>
+      <Button variant="outline" onClick={() => window.history.back()}>
+        Go back
+      </Button>
+      <Button onClick={() => void router.navigate('/')}>
+        Go home
+      </Button>
+    </div>
+  )
+}
diff --git a/web-client/src/app/router/routes.tsx b/web-client/src/app/router/routes.tsx
index 7899155..668fe74 100644
--- a/web-client/src/app/router/routes.tsx
+++ b/web-client/src/app/router/routes.tsx
@@ -7,6 +7,7 @@ import { LettersPage } from '@/features/letters'
 import { OrganizationPage } from '@/features/organization'
 import { FeedbackPage } from '@/features/feedback'
 import { HelperPage } from '@/features/helper'
+import { NotFoundPage } from '@/app/pages/NotFoundPage'
 
 export const router = createBrowserRouter([
   {
@@ -21,6 +22,7 @@ export const router = createBrowserRouter([
       { path: 'organization', element: <OrganizationPage /> },
       { path: 'feedback', element: <FeedbackPage /> },
       { path: 'helper', element: <HelperPage /> },
+      { path: '*', element: <NotFoundPage /> },
     ],
   },
 ])
diff --git a/web-client/src/app/theme/ThemeToggle.tsx b/web-client/src/app/theme/ThemeToggle.tsx
index 88281c6..0abeef4 100644
--- a/web-client/src/app/theme/ThemeToggle.tsx
+++ b/web-client/src/app/theme/ThemeToggle.tsx
@@ -1,6 +1,13 @@
 import { Monitor, Moon, Sun } from 'lucide-react'
 import { useTheme } from './useTheme'
 import { Button } from '@/components/ui/button'
+import { cn } from '@/lib/utils'
+
+const themeIconClassName = (isActive: boolean, inactiveRotationClass: string) =>
+  cn(
+    'size-4 transition-all duration-200',
+    isActive ? 'scale-100 rotate-0 opacity-100' : `absolute scale-0 ${inactiveRotationClass} opacity-0`,
+  )
 
 export function ThemeToggle() {
   const { theme, toggleTheme } = useTheme()
@@ -14,9 +21,9 @@ export function ThemeToggle() {
       title={`Toggle theme (current: ${theme})`}
       className="relative border border-border bg-background/70"
     >
-      <Sun className={`h-4 w-4 transition-all duration-200 ${theme === 'light' ? 'scale-100 rotate-0 opacity-100' : 'absolute scale-0 -rotate-90 opacity-0'}`} />
-      <Moon className={`h-4 w-4 transition-all duration-200 ${theme === 'dark' ? 'scale-100 rotate-0 opacity-100' : 'absolute scale-0 rotate-90 opacity-0'}`} />
-      <Monitor className={`h-4 w-4 transition-all duration-200 ${theme === 'system' ? 'scale-100 rotate-0 opacity-100' : 'absolute scale-0 -rotate-90 opacity-0'}`} />
+      <Sun className={themeIconClassName(theme === 'light', '-rotate-90')} />
+      <Moon className={themeIconClassName(theme === 'dark', 'rotate-90')} />
+      <Monitor className={themeIconClassName(theme === 'system', '-rotate-90')} />
     </Button>
   )
 }
diff --git a/web-client/src/components/ui/ErrorCard.tsx b/web-client/src/components/ui/ErrorCard.tsx
new file mode 100644
index 0000000..7569c37
--- /dev/null
+++ b/web-client/src/components/ui/ErrorCard.tsx
@@ -0,0 +1,50 @@
+import { AlertCircle } from 'lucide-react'
+import { Alert, AlertDescription, AlertTitle } from '@/components/ui/alert'
+import { Button } from '@/components/ui/button'
+import { Card, CardContent, CardDescription, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
+
+interface ErrorCardAction {
+  label: string
+  onClick: () => void
+}
+
+interface ErrorCardProps {
+  title: string
+  description: string
+  alertTitle?: string
+  alertDescription?: string
+  actions?: ErrorCardAction[]
+}
+
+export function ErrorCard({
+  title,
+  description,
+  alertTitle = 'Error',
+  alertDescription = 'If this keeps happening, contact support.',
+  actions = [{ label: 'Try again', onClick: () => window.location.reload() }],
+}: ErrorCardProps) {
+  return (
+    <div className="flex min-h-screen items-center justify-center bg-background p-4">
+      <Card className="w-full max-w-sm">
+        <CardHeader>
+          <CardTitle>{title}</CardTitle>
+          <CardDescription>{description}</CardDescription>
+        </CardHeader>
+        <CardContent>
+          <Alert variant="destructive">
+            <AlertCircle className="size-4" />
+            <AlertTitle>{alertTitle}</AlertTitle>
+            <AlertDescription>{alertDescription}</AlertDescription>
+          </Alert>
+        </CardContent>
+        <CardFooter className="gap-3">
+          {actions.map((action) => (
+            <Button key={action.label} className="flex-1" onClick={action.onClick}>
+              {action.label}
+            </Button>
+          ))}
+        </CardFooter>
+      </Card>
+    </div>
+  )
+}
diff --git a/web-client/src/main.tsx b/web-client/src/main.tsx
index b78d8e9..5a9254b 100644
--- a/web-client/src/main.tsx
+++ b/web-client/src/main.tsx
@@ -1,6 +1,6 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
-import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
+import { QueryClient } from '@tanstack/react-query'
 import { AxiosError } from 'axios'
 import { ThemeProvider } from '@/app/theme/ThemeProvider'
 import AuthenticatedApp from './AuthenticatedApp.tsx'
@@ -29,9 +29,7 @@ const queryClient = new QueryClient({
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
     <ThemeProvider>
-      <QueryClientProvider client={queryClient}>
-        <AuthenticatedApp />
-      </QueryClientProvider>
+      <AuthenticatedApp queryClient={queryClient} />
     </ThemeProvider>
   </StrictMode>,
 )