diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
index e75bc32..8e386e5 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
@@ -104,6 +104,8 @@
 import {Route, ViewState} from '../../../models/views/base';
 import {Model} from '../../../models/model';
 
+// TODO: Move all patterns to view model files and use the `Route` interface,
+// which will enforce using `RegExp` in its `urlPattern` property.
 const RoutePattern = {
   ROOT: '/',
 
@@ -255,7 +257,7 @@
 
   PLUGIN_SCREEN: /^\/x\/([\w-]+)\/([\w-]+)\/?/,
 
-  DOCUMENTATION_SEARCH_FILTER: '/Documentation/q/filter::filter',
+  DOCUMENTATION_SEARCH_FILTER: /^\/Documentation\/q\/filter:(.*)$/,
   DOCUMENTATION_SEARCH: /^\/Documentation\/q\/(.*)$/,
   DOCUMENTATION: /^\/Documentation(\/)?(.+)?/,
 };
@@ -1778,7 +1780,7 @@
   handleDocumentationSearchRoute(ctx: PageContext) {
     const state: DocumentationViewState = {
       view: GerritView.DOCUMENTATION_SEARCH,
-      filter: ctx.params['filter'] || null,
+      filter: ctx.params[0] ?? '',
     };
     // Note that router model view must be updated before view models.
     this.setState(state);
diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
index 35ce54c..d486f66 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
@@ -1030,5 +1030,40 @@
         screen: 'bar',
       });
     });
+
+    test('DOCUMENTATION_SEARCH*', async () => {
+      // DOCUMENTATION_SEARCH_FILTER: '/Documentation/q/filter::filter',
+      // DOCUMENTATION_SEARCH: /^\/Documentation\/q\/(.*)$/,
+      await checkRedirect(
+        '/Documentation/q/asdf',
+        '/Documentation/q/filter:asdf'
+      );
+      await checkRedirect(
+        '/Documentation/q/as%20df',
+        '/Documentation/q/filter:as%20df'
+      );
+
+      await checkUrlToState('/Documentation/q/filter:', {
+        view: GerritView.DOCUMENTATION_SEARCH,
+        filter: '',
+      });
+      await checkUrlToState('/Documentation/q/filter:asdf', {
+        view: GerritView.DOCUMENTATION_SEARCH,
+        filter: 'asdf',
+      });
+      // Percent decoding works fine. page.js decodes twice, so the only problem
+      // is having `%25` in the URL, because the first decoding pass will yield
+      // `%`, and then the second decoding pass will throw `URI malformed`.
+      await checkUrlToState('/Documentation/q/filter:as%20%2fdf', {
+        view: GerritView.DOCUMENTATION_SEARCH,
+        filter: 'as /df',
+      });
+      // We accept and process double-encoded values, but only *require* it for
+      // the percent symbol `%`.
+      await checkUrlToState('/Documentation/q/filter:as%252f%2525df', {
+        view: GerritView.DOCUMENTATION_SEARCH,
+        filter: 'as/%df',
+      });
+    });
   });
 });
diff --git a/polygerrit-ui/app/models/views/documentation.ts b/polygerrit-ui/app/models/views/documentation.ts
index b564d64..6789695 100644
--- a/polygerrit-ui/app/models/views/documentation.ts
+++ b/polygerrit-ui/app/models/views/documentation.ts
@@ -10,7 +10,7 @@
 
 export interface DocumentationViewState extends ViewState {
   view: GerritView.DOCUMENTATION_SEARCH;
-  filter?: string | null;
+  filter: string;
 }
 
 export const documentationViewModelToken = define<DocumentationViewModel>(