diff --git a/packages/docusaurus/src/client/exports/__tests__/useBaseUrl.test.tsx b/packages/docusaurus/src/client/exports/__tests__/useBaseUrl.test.tsx
index 43725ee1e1..babbadbe05 100644
--- a/packages/docusaurus/src/client/exports/__tests__/useBaseUrl.test.tsx
+++ b/packages/docusaurus/src/client/exports/__tests__/useBaseUrl.test.tsx
@@ -8,17 +8,221 @@
 import React from 'react';
 import {renderHook} from '@testing-library/react-hooks';
 import {fromPartial} from '@total-typescript/shoehorn';
-import useBaseUrl, {useBaseUrlUtils} from '../useBaseUrl';
+import useBaseUrl, {addBaseUrl, useBaseUrlUtils} from '../useBaseUrl';
 import {Context} from '../../docusaurusContext';
 import type {DocusaurusContext, FutureConfig} from '@docusaurus/types';
 import type {BaseUrlOptions} from '@docusaurus/useBaseUrl';
 
+type AddBaseUrlParams = Parameters<typeof addBaseUrl>[0];
+
 const future: FutureConfig = fromPartial({
   experimental_router: 'browser',
 });
 
 const forcePrepend = {forcePrependBaseUrl: true};
 
+// TODO migrate more tests here, it's easier to test a pure function
+describe('addBaseUrl', () => {
+  function baseTest(params: Partial<AddBaseUrlParams>) {
+    return addBaseUrl({
+      siteUrl: 'https://docusaurus.io',
+      baseUrl: '/baseUrl/',
+      url: 'hello',
+      router: 'browser',
+      ...params,
+    });
+  }
+
+  describe('with browser router', () => {
+    function test(params: {
+      url: AddBaseUrlParams['url'];
+      baseUrl: AddBaseUrlParams['baseUrl'];
+      options?: AddBaseUrlParams['options'];
+    }) {
+      return baseTest({
+        ...params,
+        router: 'browser',
+      });
+    }
+
+    it('/baseUrl/ + hello', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: 'hello',
+        }),
+      ).toBe('/baseUrl/hello');
+    });
+
+    it('/baseUrl/ + hello - absolute option', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: 'hello',
+          options: {absolute: true},
+        }),
+      ).toBe('https://docusaurus.io/baseUrl/hello');
+    });
+
+    it('/baseUrl/ + /hello', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: '/hello',
+        }),
+      ).toBe('/baseUrl/hello');
+    });
+
+    it('/baseUrl/ + /hello - absolute option', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: '/hello',
+          options: {absolute: true},
+        }),
+      ).toBe('https://docusaurus.io/baseUrl/hello');
+    });
+
+    it('/ + hello', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: 'hello',
+        }),
+      ).toBe('/hello');
+    });
+
+    it('/ + hello - absolute', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: 'hello',
+          options: {absolute: true},
+        }),
+      ).toBe('https://docusaurus.io/hello');
+    });
+
+    it('/ + /hello', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: '/hello',
+        }),
+      ).toBe('/hello');
+    });
+
+    it('/ + /hello - absolute', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: '/hello',
+          options: {absolute: true},
+        }),
+      ).toBe('https://docusaurus.io/hello');
+    });
+  });
+
+  describe('with hash router', () => {
+    function test(params: {
+      url: AddBaseUrlParams['url'];
+      baseUrl: AddBaseUrlParams['baseUrl'];
+      options?: AddBaseUrlParams['options'];
+    }) {
+      return baseTest({
+        ...params,
+        router: 'hash',
+      });
+    }
+
+    it('/baseUrl/ + hello', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: 'hello',
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/baseUrl/ + hello - absolute option', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: 'hello',
+          options: {absolute: true},
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/baseUrl/ + /hello', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: '/hello',
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/baseUrl/ + /hello - absolute option', () => {
+      expect(
+        test({
+          baseUrl: '/baseUrl/',
+          url: '/hello',
+          options: {absolute: true},
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/ + hello', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: 'hello',
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/ + hello - absolute', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: 'hello',
+          options: {absolute: true},
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/ + /hello', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: 'hello',
+          options: {absolute: true},
+        }),
+      ).toBe('./hello');
+    });
+
+    it('/ + /hello - absolute', () => {
+      expect(
+        test({
+          baseUrl: '/',
+          url: 'hello',
+          options: {absolute: true},
+        }),
+      ).toBe('./hello');
+    });
+  });
+
+  /*
+
+src
+:
+"img/docusaurus.svg"
+srcDark
+:
+"img/docusaurus_keytar.svg"
+   */
+});
+
 describe('useBaseUrl', () => {
   const createUseBaseUrlMock =
     (context: DocusaurusContext) => (url: string, options?: BaseUrlOptions) =>
diff --git a/packages/docusaurus/src/client/exports/useBaseUrl.ts b/packages/docusaurus/src/client/exports/useBaseUrl.ts
index f9d17b93a2..609e9c8447 100644
--- a/packages/docusaurus/src/client/exports/useBaseUrl.ts
+++ b/packages/docusaurus/src/client/exports/useBaseUrl.ts
@@ -11,7 +11,7 @@ import {hasProtocol} from './isInternalUrl';
 import type {BaseUrlOptions, BaseUrlUtils} from '@docusaurus/useBaseUrl';
 import type {RouterType} from '@docusaurus/types';
 
-function addBaseUrl({
+export function addBaseUrl({
   siteUrl,
   baseUrl,
   url,
@@ -30,9 +30,11 @@ function addBaseUrl({
     return url;
   }
 
-  // TODO temp hack
-  if (router === 'hash' && url.startsWith('/')) {
-    return `.${url}`;
+  // TODO hash router + /baseUrl/ is unlikely to work well in all situations
+  // This will support most cases, but not all
+  // See https://github.com/facebook/docusaurus/pull/9859
+  if (router === 'hash') {
+    return url.startsWith('/') ? `.${url}` : `./${url}`;
   }
 
   if (forcePrependBaseUrl) {