diff --git a/apps/files/src/main.ts b/apps/files/src/main.ts
index bc76d8bcffcfc..598053265b811 100644
--- a/apps/files/src/main.ts
+++ b/apps/files/src/main.ts
@@ -11,9 +11,11 @@ import SettingsModel from './models/Setting.ts'
 import router from './router/router.ts'
 import RouterService from './services/RouterService.ts'
 import SettingsService from './services/Settings.js'
+import registerUploadBeforeUnload from './services/UploadBeforeUnload.ts'
 import { getPinia } from './store/index.ts'
 
 __webpack_nonce__ = getCSPNonce()
+registerUploadBeforeUnload()
 
 // Init private and public Files namespace
 window.OCA.Files = window.OCA.Files ?? {}
diff --git a/apps/files/src/services/UploadBeforeUnload.spec.ts b/apps/files/src/services/UploadBeforeUnload.spec.ts
new file mode 100644
index 0000000000000..dc0874719b676
--- /dev/null
+++ b/apps/files/src/services/UploadBeforeUnload.spec.ts
@@ -0,0 +1,78 @@
+/*!
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+const getUploader = vi.hoisted(() => vi.fn())
+const t = vi.hoisted(() => vi.fn())
+
+vi.mock('@nextcloud/upload', () => ({
+	getUploader,
+	UploaderStatus: {
+		IDLE: 0,
+		UPLOADING: 1,
+		PAUSED: 2,
+	},
+}))
+
+vi.mock('@nextcloud/l10n', () => ({
+	t,
+}))
+
+describe('registerUploadBeforeUnload', () => {
+	beforeEach(() => {
+		vi.resetModules()
+		vi.clearAllMocks()
+	})
+
+	async function getBeforeUnloadHandler(): Promise<(event: BeforeUnloadEvent) => void> {
+		const addListener = vi.spyOn(window, 'addEventListener')
+		const { default: register } = await import('./UploadBeforeUnload.ts')
+		register()
+		const call = addListener.mock.calls.find((c) => c[0] === 'beforeunload')
+		expect(call).toBeDefined()
+		addListener.mockRestore()
+		return call![1] as (event: BeforeUnloadEvent) => void
+	}
+
+	it('does not prevent unload or translate when uploader is idle', async () => {
+		getUploader.mockReturnValue({ info: { status: 0 } })
+		const handler = await getBeforeUnloadHandler()
+		const event = new Event('beforeunload') as BeforeUnloadEvent
+		const preventDefault = vi.spyOn(event, 'preventDefault')
+		handler(event)
+		expect(preventDefault).not.toHaveBeenCalled()
+		expect(t).not.toHaveBeenCalled()
+	})
+
+	it.each([
+		['uploading', 1],
+		['paused', 2],
+	] as const)('sets leave warning when uploader is %s', async (_label, status) => {
+		t.mockReturnValue('leave-warning')
+		getUploader.mockReturnValue({ info: { status } })
+		const handler = await getBeforeUnloadHandler()
+		const event = new Event('beforeunload') as BeforeUnloadEvent
+		const preventDefault = vi.spyOn(event, 'preventDefault')
+		handler(event)
+		expect(preventDefault).toHaveBeenCalled()
+		expect(t).toHaveBeenCalledWith(
+			'files',
+			'File uploads are still in progress. Leaving the page will cancel them.',
+		)
+	})
+
+	it('adds only one beforeunload listener when register is called multiple times', async () => {
+		getUploader.mockReturnValue({ info: { status: 0 } })
+		const addListener = vi.spyOn(window, 'addEventListener')
+		const { default: register } = await import('./UploadBeforeUnload.ts')
+		register()
+		register()
+		register()
+		const beforeUnloadCalls = addListener.mock.calls.filter((c) => c[0] === 'beforeunload')
+		expect(beforeUnloadCalls).toHaveLength(1)
+		addListener.mockRestore()
+	})
+})
diff --git a/apps/files/src/services/UploadBeforeUnload.ts b/apps/files/src/services/UploadBeforeUnload.ts
new file mode 100644
index 0000000000000..f2a331842886b
--- /dev/null
+++ b/apps/files/src/services/UploadBeforeUnload.ts
@@ -0,0 +1,26 @@
+import { t } from '@nextcloud/l10n'
+import { getUploader, UploaderStatus } from '@nextcloud/upload'
+
+let registered = false
+function onBeforeUnload(event: BeforeUnloadEvent): void {
+	const uploader = getUploader()
+	if (uploader.info.status === UploaderStatus.IDLE) {
+		return
+	}
+	event.preventDefault()
+	event.returnValue = t(
+		'files',
+		'File uploads are still in progress. Leaving the page will cancel them.',
+	)
+}
+
+/**
+* Warn before closing or navigating away while uploads are running or paused.
+*/
+export default function registerUploadBeforeUnload(): void {
+	if (registered) {
+		return
+	}
+	registered = true
+	window.addEventListener('beforeunload', onBeforeUnload)
+}
diff --git a/apps/files_sharing/src/init-public.ts b/apps/files_sharing/src/init-public.ts
index b59389cb5c22e..5656f892ff965 100644
--- a/apps/files_sharing/src/init-public.ts
+++ b/apps/files_sharing/src/init-public.ts
@@ -10,6 +10,7 @@ import { emit, subscribe, unsubscribe } from '@nextcloud/event-bus'
 import { getNavigation } from '@nextcloud/files'
 import { loadState } from '@nextcloud/initial-state'
 import RouterService from '../../files/src/services/RouterService.ts'
+import registerUploadBeforeUnload from '../../files/src/services/UploadBeforeUnload.ts'
 import registerFileDropView from './files_views/publicFileDrop.ts'
 import registerPublicFileShareView from './files_views/publicFileShare.ts'
 import registerPublicShareView from './files_views/publicShare.ts'
@@ -19,6 +20,7 @@ import logger from './services/logger.ts'
 registerFileDropView()
 registerPublicShareView()
 registerPublicFileShareView()
+registerUploadBeforeUnload()
 
 // Get the current view from state and set it active
 const view = loadState<string>('files_sharing', 'view')