diff --git a/__tests__/sidebar/sidebarTab.spec.ts b/__tests__/sidebar/sidebarTab.spec.ts
new file mode 100644
index 000000000..4a1dd461e
--- /dev/null
+++ b/__tests__/sidebar/sidebarTab.spec.ts
@@ -0,0 +1,161 @@
+/**
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { getSidebarTabs, ISidebarTab, registerSidebarTab } from '../../lib/sidebar'
+
+// missing in JSDom but supported by every browser!
+import 'css.escape'
+
+describe('Sidebar tabs', () => {
+	beforeEach(() => {
+		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 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(), iconSvg: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have an valid SVG icon]')
+		})
+
+		it('fails with invalid SVG icon', () => {
+			expect(
+				() => registerSidebarTab({ ...getExampleTab(), iconSvg: '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]')
+		})
+
+		it('fails with missing "setActive" method', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), setActive: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tabs need to have a "setActive" method]')
+		})
+
+		it.for(['mount', 'unmount', 'update'])('fails with missing lifecylce methods', (method) => {
+			expect(
+				() => registerSidebarTab({ ...getExampleTab(), [method]: undefined }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: Sidebar tab is missing a required lifecycle method]')
+		})
+
+		it('works without specifying a scroll listener', () => {
+			expect(
+				() => registerSidebarTab({ ...getExampleTab(), onScrollBottomReached: undefined }),
+			).not.toThrow()
+		})
+
+		it('fails with an invalid scroll listener', () => {
+			expect(
+				// @ts-expect-error mocking for testing
+				() => registerSidebarTab({ ...getExampleTab(), onScrollBottomReached: 'not a method' }),
+			).toThrowErrorMatchingInlineSnapshot('[Error: "onScrollBottomReached" of the sidebar tab needs to be a function]')
+		})
+	})
+})
+
+/**
+ * Get a very basic mock of a sidebar tab
+ */
+function getExampleTab(): ISidebarTab {
+	return {
+		id: 'example-tab',
+		displayName: 'Example',
+		enabled: vi.fn(),
+		iconSvg: '<svg><circle r="45" cx="50" cy="50" fill="red" /></svg>',
+		order: 0,
+		mount: vi.fn(),
+		unmount: vi.fn(),
+		update: vi.fn(),
+		setActive: vi.fn(),
+		onScrollBottomReached: vi.fn(),
+	}
+}
diff --git a/lib/index.ts b/lib/index.ts
index cd16c35d9..3f97ec971 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -10,6 +10,7 @@ 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'
 
 // Legacy export of dav utils
diff --git a/lib/sidebar/index.ts b/lib/sidebar/index.ts
new file mode 100644
index 000000000..3ca7d2bf1
--- /dev/null
+++ b/lib/sidebar/index.ts
@@ -0,0 +1,165 @@
+/*
+ * SPDX-FileCopyrightText: 2025 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import type { IFolder, INode } from '../node/index.ts'
+
+import isSvg from 'is-svg'
+import logger from '../utils/logger.ts'
+
+interface SidebarUpdateContext {
+	/**
+	 * The currently selected node for which the sidebar is shown.
+	 */
+	node: INode
+
+	/**
+	 * The parent of the current selected node.
+	 * This is not necessarily the real parent folder of the node in means of the real filesystem tree,
+	 * but rather the parent folder in the current view of the files app.
+	 */
+	parent: IFolder
+}
+
+/**
+ * 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.
+	 */
+	iconSvg: string
+
+	/**
+	 * The order of this tab.
+	 * Use a low number to make this tab ordered in front.
+	 */
+	order: number
+
+	/**
+	 * Callback to check if the sidebar tab should be shown for the selected node.
+	 *
+	 * @param node - The currently selected node
+	 */
+	enabled: (node: INode) => boolean
+
+	/**
+	 * Called by the files app if this tab has become the active tab or was deactivated.
+	 *
+	 * @param active - The new active state of this tab
+	 */
+	setActive: (active: boolean) => void | Promise<void>
+
+	/**
+	 * The lifecylce method for mounting the sidebar tab onto the sidebar.
+	 *
+	 * @param el - The element to mount the sidebar tab to
+	 * @param context - The current sidebar context
+	 */
+	mount: (el: HTMLElement, context: SidebarUpdateContext) => void | Promise<void>
+
+	/**
+	 * The lifecycle method for updating the sidebar tab.
+	 * This is called if the currently selected node changes.
+	 *
+	 * @param context - The current sidebar context
+	 */
+	update: (context: SidebarUpdateContext) => void | Promise<void>
+
+	/**
+	 * The lifecycle method for unmounting the sidebar tab.
+	 * This is called if the sidebar is unmounted from the files app and thus the sidebar tab needs to do its cleanup and unmounting.
+	 */
+	unmount: () => void | Promise<void>
+
+	/**
+	 * Called when the bottom of the sidebar was reached during scrolling.
+	 */
+	onScrollBottomReached?: () => void | Promise<void>
+}
+
+/**
+ * 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.displayName || typeof tab.displayName !== 'string') {
+		throw new Error('Sidebar tabs need to have a name set')
+	}
+
+	if (typeof tab.iconSvg !== 'string' || !isSvg(tab.iconSvg)) {
+		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')
+	}
+
+	if (typeof tab.setActive !== 'function') {
+		throw new Error('Sidebar tabs need to have a "setActive" method')
+	}
+
+	if (typeof tab.mount !== 'function'
+		|| typeof tab.update !== 'function'
+		|| typeof tab.unmount !== 'function'
+	) {
+		throw new Error('Sidebar tab is missing a required lifecycle method')
+	}
+
+	if (tab.onScrollBottomReached && typeof tab.onScrollBottomReached !== 'function') {
+		throw new Error('"onScrollBottomReached" of the sidebar tab needs to be a function')
+	}
+}
diff --git a/lib/window.d.ts b/lib/window.d.ts
index 0af2eb4ea..fbd357f05 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 aa6719a16..2a6a2053b 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -29,6 +29,7 @@
         "@nextcloud/vite-config": "^2.4.0",
         "@types/node": "^24.2.1",
         "@vitest/coverage-istanbul": "^3.2.4",
+        "css.escape": "^1.5.1",
         "fast-xml-parser": "^5.2.5",
         "jsdom": "^26.1.0",
         "tslib": "^2.8.1",
@@ -4406,6 +4407,13 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "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 a2c370203..eacd024eb 100644
--- a/package.json
+++ b/package.json
@@ -68,6 +68,7 @@
     "@nextcloud/vite-config": "^2.4.0",
     "@types/node": "^24.2.1",
     "@vitest/coverage-istanbul": "^3.2.4",
+    "css.escape": "^1.5.1",
     "fast-xml-parser": "^5.2.5",
     "jsdom": "^26.1.0",
     "tslib": "^2.8.1",