diff --git a/.eslintrc.json b/.eslintrc.json
index 64afe8fb6..8a95224bc 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -1,5 +1,13 @@
 {
 	"extends": [
 		"@nextcloud/eslint-config/typescript"
+	],
+	"overrides": [
+		{
+			"files": ["**.spec.*"],
+			"rules": {
+				"no-console": "off"
+			}
+		}
 	]
 }
diff --git a/__tests__/sidebar/sidebarTab.spec.ts b/__tests__/sidebar/sidebarTab.spec.ts
new file mode 100644
index 000000000..503a907d3
--- /dev/null
+++ b/__tests__/sidebar/sidebarTab.spec.ts
@@ -0,0 +1,172 @@
+/**
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import type { IFolder, INode, IView } from '../../lib/index.ts'
+
+import { beforeEach, describe, expect, it, Mock, vi } from 'vitest'
+import { getSidebarTabs, ISidebarTab, registerSidebarTab } from '../../lib/sidebar'
+// missing in JSDom but supported by every browser!
+import 'css.escape'
+
+class SidebarTabMock extends HTMLElement {
+
+	node?: INode
+	folder?: IFolder
+	view?: IView
+
+	public setActive(active: boolean) {
+		console.log('setActive', active)
+	}
+
+}
+
+describe('Sidebar tabs', () => {
+	let getCustomElementsSpy: Mock
+
+	beforeEach(() => {
+		vi.restoreAllMocks()
+		getCustomElementsSpy = vi.spyOn(window.customElements, 'get')
+			.mockImplementation(() => SidebarTabMock)
+		delete window._nc_files_sidebar_tabs
+	})
+
+	it('can register a tab', () => {
+		const tab = getExampleTab()
+
+		registerSidebarTab(tab)
+		expect(window._nc_files_sidebar_tabs).toBeInstanceOf(Map)
+		expect(window._nc_files_sidebar_tabs!.has(tab.id)).toBe(true)
+		expect(window._nc_files_sidebar_tabs!.get(tab.id)).toBe(tab)
+	})
+
+	it('can fetch empty list of sidebar tabs', () => {
+		expect(getSidebarTabs()).toBeInstanceOf(Array)
+		expect(getSidebarTabs()).toHaveLength(0)
+	})
+
+	it('can fetch list of sidebar tabs', () => {
+		registerSidebarTab(getExampleTab())
+		registerSidebarTab({ ...getExampleTab(), id: 'another-example' })
+
+		expect(getSidebarTabs()).toBeInstanceOf(Array)
+		expect(getSidebarTabs()).toHaveLength(2)
+	})
+
+	it('only registeres same id once', () => {
+		const consoleSpy = vi.spyOn(console, 'warn')
+		consoleSpy.mockImplementationOnce(() => {})
+
+		registerSidebarTab(getExampleTab())
+		registerSidebarTab(getExampleTab())
+		expect(consoleSpy).toHaveBeenCalledOnce()
+		expect(getSidebarTabs()).toHaveLength(1)
+	})
+
+	describe('Tab validation', () => {
+		it('fails with an invalid parameter', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab(getExampleTab),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tab is not an object]')
+		})
+
+		it('fails with missing id', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), id: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have an id conforming to the HTML id attribute specifications]')
+		})
+
+		it('fails with non conforming id', () => {
+			expect(
+				() => registerSidebarTab({ ...getExampleTab(), id: 'this is invalid' }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have an id conforming to the HTML id attribute specifications]')
+		})
+
+		it('fails with missing tagName name', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), tagName: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have the tagName name set]')
+		})
+
+		it('fails with invalid tagName name', () => {
+			expect(() => registerSidebarTab({ ...getExampleTab(), tagName: 'MyAppSidebarTab' }))
+				.toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs tagName name is invalid]')
+		})
+
+		it('fails with non registered element', () => {
+			getCustomElementsSpy.mockImplementationOnce(() => undefined)
+			expect(() => registerSidebarTab(getExampleTab())).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tab element not registered]')
+		})
+
+		it('fails with invalid custom element', () => {
+			getCustomElementsSpy.mockImplementationOnce(() => HTMLElement)
+			expect(() => registerSidebarTab(getExampleTab())).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tab elements must have the `setActive` method]')
+		})
+
+		it('fails with missing name', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), displayName: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have a name set]')
+		})
+
+		it('fails with invalid name', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), displayName: 1234 }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have a name set]')
+		})
+
+		it('fails with missing icon', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), iconSvgInline: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have an valid SVG icon]')
+		})
+
+		it('fails with invalid SVG icon', () => {
+			expect(
+				() => registerSidebarTab({ ...getExampleTab(), iconSvgInline: 'icon-group' }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have an valid SVG icon]')
+		})
+
+		it('fails with missing order', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), order: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have a numeric order set]')
+		})
+
+		it('fails with invalid order', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), order: '3' }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have a numeric order set]')
+		})
+
+		it('fails with missing "enabled" method', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), enabled: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have an "enabled" method]')
+		})
+	})
+})
+
+/**
+ * Get a very basic mock of a sidebar tab
+ */
+function getExampleTab(): ISidebarTab {
+	return {
+		id: 'example-tab',
+		displayName: 'Example',
+		tagName: 'example_app-files-sidebar-tab',
+		enabled: vi.fn(),
+		iconSvgInline: '<svg><circle r="45" cx="50" cy="50" fill="red" /></svg>',
+		order: 0,
+	}
+}
diff --git a/__tests__/view.spec.ts b/__tests__/view.spec.ts
index 04291c128..969c6a015 100644
--- a/__tests__/view.spec.ts
+++ b/__tests__/view.spec.ts
@@ -182,6 +182,9 @@ describe('View creation', () => {
 	})
 })
 
+/**
+ * Creates a mock View and its associated Folder for testing purposes.
+ */
 export function mockView() {
 	const folder = new Folder({
 		source: 'https://example.org/dav/files/admin/',
diff --git a/lib/index.ts b/lib/index.ts
index 6327ae270..69c32d997 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -12,4 +12,5 @@ export * from './navigation/index.ts'
 export * from './newMenu/index.ts'
 export * from './node/index.ts'
 export * from './permissions.ts'
+export * from './sidebar/index.ts'
 export * from './utils/index.ts'
diff --git a/lib/sidebar/index.ts b/lib/sidebar/index.ts
new file mode 100644
index 000000000..0ef3afe2f
--- /dev/null
+++ b/lib/sidebar/index.ts
@@ -0,0 +1,163 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import type { IView } from '../navigation/view.ts'
+import type { IFolder, INode } from '../node/index.ts'
+
+import isSvg from 'is-svg'
+import logger from '../utils/logger.ts'
+
+export interface ISidebarContext {
+	/**
+	 * The active node in the sidebar
+	 */
+	node: INode
+
+	/**
+	 * The current open folder in the files app
+	 */
+	folder: IFolder
+
+	/**
+	 * The currently active view
+	 */
+	view: IView
+}
+
+/**
+ * This component describes the custom web component that should be registered for a sidebar tab.
+ *
+ * @see https://developer.mozilla.org/en-US/docs/Web/API/Web_components
+ * @see https://vuejs.org/guide/extras/web-components#building-custom-elements-with-vue
+ */
+export interface SidebarComponent extends HTMLElement, ISidebarContext {
+	/**
+	 * This method is called by the files app if the sidebar tab state changes.
+	 *
+	 * @param active - The new active state
+	 */
+	setActive(active: boolean): Promise<void>
+}
+
+/**
+ * Implementation of a custom sidebar tab within the files app.
+ */
+export interface ISidebarTab {
+	/**
+	 * Unique id of the sidebar tab.
+	 * This has to conform to the HTML id attribute specification.
+	 */
+	id: string
+
+	/**
+	 * The localized name of the sidebar tab.
+	 */
+	displayName: string
+
+	/**
+	 * The icon, as SVG, of the sidebar tab.
+	 */
+	iconSvgInline: string
+
+	/**
+	 * The order of this tab.
+	 * Use a low number to make this tab ordered in front.
+	 */
+	order: number
+
+	/**
+	 * The tag name of the web component.
+	 * The web component must already be registered under that tag name with `CustomElementRegistry.define()`.
+	 *
+	 * To avoid name clashes the name has to start with your appid (e.g. `your_app`).
+	 * So in addition with the web component naming rules a good name would be `your_app-files-sidebar-tab`.
+	 */
+	tagName: string
+
+	/**
+	 * Callback to check if the sidebar tab should be shown for the selected node.
+	 *
+	 * @param context - The current context of the files app
+	 */
+	enabled: (context: ISidebarContext) => boolean
+}
+
+/**
+ * Register a new sidebar tab for the files app.
+ *
+ * @param tab - The sidebar tab to register
+ * @throws If the provided tab is not a valid sidebar tab and thus cannot be registered.
+ */
+export function registerSidebarTab(tab: ISidebarTab): void {
+	validateSidebarTab(tab)
+
+	window._nc_files_sidebar_tabs ??= new Map<string, ISidebarTab>()
+	if (window._nc_files_sidebar_tabs.has(tab.id)) {
+		logger.warn(`Sidebar tab with id "${tab.id}" already registered. Skipping.`)
+		return
+	}
+	window._nc_files_sidebar_tabs.set(tab.id, tab)
+	logger.debug(`New sidebar tab with id "${tab.id}" registered.`)
+}
+
+/**
+ * Get all currently registered sidebar tabs.
+ */
+export function getSidebarTabs(): ISidebarTab[] {
+	if (window._nc_files_sidebar_tabs) {
+		return [...window._nc_files_sidebar_tabs.values()]
+	}
+	return []
+}
+
+/**
+ * Check if a given sidebar tab objects implements all necessary fields.
+ *
+ * @param tab - The sidebar tab to validate
+ */
+function validateSidebarTab(tab: ISidebarTab): void {
+	if (typeof tab !== 'object') {
+		throw new Error('Sidebar tab is not an object')
+	}
+
+	if (!tab.id || (typeof tab.id !== 'string') || tab.id !== CSS.escape(tab.id)) {
+		throw new Error('Sidebar tabs need to have an id conforming to the HTML id attribute specifications')
+	}
+
+	if (!tab.tagName || typeof tab.tagName !== 'string') {
+		throw new Error('Sidebar tabs need to have the tagName name set')
+	}
+
+	if (!tab.tagName.match(/^[a-z][a-z0-9-_]+$/)) {
+		throw new Error('Sidebar tabs tagName name is invalid')
+	}
+
+	if (!tab.displayName || typeof tab.displayName !== 'string') {
+		throw new Error('Sidebar tabs need to have a name set')
+	}
+
+	if (typeof tab.iconSvgInline !== 'string' || !isSvg(tab.iconSvgInline)) {
+		throw new Error('Sidebar tabs need to have an valid SVG icon')
+	}
+
+	if (typeof tab.order !== 'number') {
+		throw new Error('Sidebar tabs need to have a numeric order set')
+	}
+
+	if (typeof tab.enabled !== 'function') {
+		throw new Error('Sidebar tabs need to have an "enabled" method')
+	}
+
+	// now check the custom element constructor
+	const tagConstructor = window.customElements.get(tab.tagName)
+	if (!tagConstructor) {
+		throw new Error('Sidebar tab element not registered')
+	}
+
+	if (!('setActive' in tagConstructor.prototype)) {
+		// we cannot check properties like `node` or `view` because those are not necessarily defined in the prototype.
+		throw new Error('Sidebar tab elements must have the `setActive` method')
+	}
+}
diff --git a/lib/window.d.ts b/lib/window.d.ts
index 32bb6a0cf..84b1c5007 100644
--- a/lib/window.d.ts
+++ b/lib/window.d.ts
@@ -13,9 +13,8 @@ import type {
 	NewMenu,
 } from './index.ts'
 
-import type {
-	DavProperty,
-} from './dav/index.ts'
+import type { DavProperty } from './dav/index.ts'
+import type { ISidebarTab } from './sidebar/index.ts'
 
 export {}
 
@@ -30,6 +29,7 @@ declare global {
 		_nc_newfilemenu?: NewMenu
 		_nc_navigation?: Navigation
 		_nc_filelist_filters?: Map<string, IFileListFilter>
+		_nc_files_sidebar_tabs?: Map<string, ISidebarTab>
 
 		_oc_config?: {
 			forbidden_filenames_characters: string[]
diff --git a/package-lock.json b/package-lock.json
index b4378598d..a2b961597 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -29,6 +29,7 @@
         "@nextcloud/vite-config": "^2.5.2",
         "@types/node": "^25.0.0",
         "@vitest/coverage-istanbul": "^4.0.15",
+        "css.escape": "^1.5.1",
         "fast-xml-parser": "^5.3.2",
         "jsdom": "^27.3.0",
         "tslib": "^2.8.1",
@@ -4705,6 +4706,13 @@
         "node": "^10 || ^12.20.0 || ^14.13.0 || >=15.0.0"
       }
     },
+    "node_modules/css.escape": {
+      "version": "1.5.1",
+      "resolved": "https://registry.npmjs.org/css.escape/-/css.escape-1.5.1.tgz",
+      "integrity": "sha512-YUifsXXuknHlUsmlgyY0PKzgPOr7/FjCePfHNt0jxm83wHZi44VDMQ7/fGNkjY3/jV1MC+1CmZbaHzugyeRtpg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/cssesc": {
       "version": "3.0.0",
       "resolved": "https://registry.npmjs.org/cssesc/-/cssesc-3.0.0.tgz",
diff --git a/package.json b/package.json
index e59fb7ee6..5f1f1dcd3 100644
--- a/package.json
+++ b/package.json
@@ -73,6 +73,7 @@
     "@nextcloud/vite-config": "^2.5.2",
     "@types/node": "^25.0.0",
     "@vitest/coverage-istanbul": "^4.0.15",
+    "css.escape": "^1.5.1",
     "fast-xml-parser": "^5.3.2",
     "jsdom": "^27.3.0",
     "tslib": "^2.8.1",