diff --git a/frontend/src/App.vue b/frontend/src/App.vue
index 08688b1bb..760c18edc 100644
--- a/frontend/src/App.vue
+++ b/frontend/src/App.vue
@@ -61,6 +61,7 @@ import {useAuthStore} from '@/stores/auth'
 import {useBaseStore} from '@/stores/base'
 
 import {useColorScheme} from '@/composables/useColorScheme'
+import {useTimeTrackingFavicon} from '@/composables/useTimeTrackingFavicon'
 import {useBodyClass} from '@/composables/useBodyClass'
 import QuickAddOverlay from '@/components/quick-actions/QuickAddOverlay.vue'
 import AddToHomeScreen from '@/components/home/AddToHomeScreen.vue'
@@ -107,6 +108,7 @@ watch(accountDeletionConfirm, async (accountDeletionConfirm) => {
 
 setLanguage(authStore.settings.language ?? DEFAULT_LANGUAGE)
 useColorScheme()
+useTimeTrackingFavicon()
 </script>
 
 <style src="@/styles/tailwind.css" />
diff --git a/frontend/src/composables/useTimeTrackingFavicon.ts b/frontend/src/composables/useTimeTrackingFavicon.ts
new file mode 100644
index 000000000..0b0a819c7
--- /dev/null
+++ b/frontend/src/composables/useTimeTrackingFavicon.ts
@@ -0,0 +1,88 @@
+import {watch} from 'vue'
+import {createSharedComposable, tryOnMounted} from '@vueuse/core'
+import {storeToRefs} from 'pinia'
+
+import {useTimeTrackingStore} from '@/stores/timeTracking'
+
+const FAVICON_SIZE = 32
+// Drawn from a PNG rather than the .ico because ICO decoding into a canvas is
+// unreliable across browsers.
+const BASE_FAVICON = '/images/icons/favicon-32x32.png'
+const DOT_COLOR = '#ff4136'
+
+function getFaviconLink(): HTMLLinkElement | null {
+	return document.querySelector<HTMLLinkElement>('link[rel="icon"]')
+}
+
+// Marks the favicon with a small red dot in the lower left corner while a timer
+// is running, so an active time tracking session is visible even when the tab
+// isn't focused.
+export const useTimeTrackingFavicon = createSharedComposable(() => {
+	const {hasActiveTimer} = storeToRefs(useTimeTrackingStore())
+
+	const link = getFaviconLink()
+	const originalHref = link?.getAttribute('href') ?? '/favicon.ico'
+
+	let baseImage: HTMLImageElement | null = null
+
+	function loadBaseImage(): Promise<HTMLImageElement> {
+		if (baseImage?.complete) {
+			return Promise.resolve(baseImage)
+		}
+		return new Promise((resolve, reject) => {
+			const img = new Image()
+			img.addEventListener('load', () => {
+				baseImage = img
+				resolve(img)
+			})
+			img.addEventListener('error', reject)
+			img.src = BASE_FAVICON
+		})
+	}
+
+	async function drawBadgedFavicon() {
+		const targetLink = getFaviconLink()
+		if (targetLink === null) {
+			return
+		}
+
+		const img = await loadBaseImage()
+		const canvas = document.createElement('canvas')
+		canvas.width = FAVICON_SIZE
+		canvas.height = FAVICON_SIZE
+		const ctx = canvas.getContext('2d')
+		if (ctx === null) {
+			return
+		}
+
+		ctx.drawImage(img, 0, 0, FAVICON_SIZE, FAVICON_SIZE)
+
+		const radius = FAVICON_SIZE * 0.28
+		const cx = radius
+		const cy = FAVICON_SIZE - radius
+		ctx.beginPath()
+		ctx.arc(cx, cy, radius, 0, Math.PI * 2)
+		ctx.fillStyle = DOT_COLOR
+		ctx.fill()
+
+		targetLink.href = canvas.toDataURL('image/png')
+	}
+
+	function restoreFavicon() {
+		const targetLink = getFaviconLink()
+		if (targetLink !== null) {
+			targetLink.href = originalHref
+		}
+	}
+
+	function update(active: boolean) {
+		if (active) {
+			void drawBadgedFavicon()
+			return
+		}
+		restoreFavicon()
+	}
+
+	watch(hasActiveTimer, update, {flush: 'post'})
+	tryOnMounted(() => update(hasActiveTimer.value))
+})