diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7b790f5..f13d30b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -23,6 +23,19 @@ skip versions are the composition of each intervening adjacent
 release's steps in order — no A-to-B path is pre-computed beyond
 that.
 
+## 0.52.3 — 2026-06-09
+
+**Patch — unmatched routes render a 404 instead of a blank page or a
+silent redirect home (no operator action required).**
+
+- **The top-level catch-all redirected any unknown path to `/`**, and the
+  nested `/admin/*`, `/p/:projectId/*`, and `/docs/*` route groups had no
+  catch-all (so an invalid subpath rendered an empty pane). Added a shared
+  `NotFound` 404 page and wired it into all four route groups, so a bad or
+  typo'd URL — including a stale `/admin/users/allowlist/graduation`-style
+  path — now shows a clear "page not found" with a link back to the catalog.
+  (Client-side 404 UI; the SPA still serves over HTTP 200, as before.)
+
 ## 0.52.2 — 2026-06-09
 
 **Patch — admin sidebar nav links no longer accumulate URL segments (no
diff --git a/VERSION b/VERSION
index fbb7b19..d572c9b 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.52.2
\ No newline at end of file
+0.52.3
\ No newline at end of file
diff --git a/frontend/package.json b/frontend/package.json
index 0a31e1c..7c77605 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.52.2",
+  "version": "0.52.3",
   "type": "module",
   "scripts": {
     "dev": "vite",
diff --git a/frontend/src/App.css b/frontend/src/App.css
index b8901f5..3b09763 100644
--- a/frontend/src/App.css
+++ b/frontend/src/App.css
@@ -2687,3 +2687,19 @@ select:focus-visible,
   align-items: flex-start;
   gap: 12px;
 }
+
+/* 404 / not-found page (client-side, for unmatched routes) */
+.not-found {
+  max-width: 32rem;
+  margin: 4rem auto;
+  padding: 0 1.5rem;
+  text-align: center;
+}
+.not-found-code {
+  font-size: 3.5rem;
+  line-height: 1;
+  margin: 0 0 0.5rem;
+  color: var(--c-gray-900, #111);
+}
+.not-found-message { color: var(--c-gray-700, #444); margin: 0 0 1.25rem; }
+.not-found-actions a { color: var(--c-gray-900, #111); text-decoration: underline; }
diff --git a/frontend/src/App.jsx b/frontend/src/App.jsx
index aabfa33..63e961d 100644
--- a/frontend/src/App.jsx
+++ b/frontend/src/App.jsx
@@ -7,6 +7,7 @@ import { brandTitle } from './lib/brand'
 import { entryPath, proposalPath, DEFAULT_COLLECTION } from './lib/entryPaths'
 import { useDeployment } from './context/DeploymentProvider'
 import ProjectLayout from './components/ProjectLayout.jsx'
+import NotFound from './components/NotFound.jsx'
 import Directory from './components/Directory.jsx'
 import ProjectSwitcher from './components/ProjectSwitcher.jsx'
 import Catalog from './components/Catalog.jsx'
@@ -381,13 +382,14 @@ export default function App() {
                   <Route path="c/:collectionId/e/:slug" element={<RFCView viewer={viewer} />} />
                   <Route path="c/:collectionId/e/:slug/pr/:prNumber" element={<PRView viewer={viewer} />} />
                   <Route path="c/:collectionId/proposals/:prNumber" element={<ProposalView viewer={viewer} onChange={() => setCatalogVersion(v => v + 1)} />} />
+                  <Route path="*" element={<NotFound />} />
                 </Routes>
               </main>
             </ProjectLayout>
           } />
-          {/* Any other path (incl. the retired bare-slug corpus URLs that
-              somehow reach the SPA) lands on the deployment landing. */}
-          <Route path="*" element={<Navigate to="/" replace />} />
+          {/* Any unmatched path is a real 404 — surface it rather than
+              silently bouncing home, so a bad/typo'd URL is visible. */}
+          <Route path="*" element={<NotFound />} />
         </Routes>
       </div>
       {(proposeOpen || proposeParam != null) && viewer && (
@@ -507,6 +509,7 @@ function DocsWithSidebar({ viewer }) {
               client-side redirect to the first configured spec. */}
           <Route path="specs" element={<DocsSpecsIndex />} />
           <Route path="specs/:name" element={<DocsSpec />} />
+          <Route path="*" element={<NotFound />} />
         </Route>
       </Routes>
     </main>
diff --git a/frontend/src/components/Admin.jsx b/frontend/src/components/Admin.jsx
index a08c504..21fcdd8 100644
--- a/frontend/src/components/Admin.jsx
+++ b/frontend/src/components/Admin.jsx
@@ -29,6 +29,7 @@ import {
 } from '../api.js'
 import { EVENTS, track } from '../lib/analytics.js'
 import { entryPath, useProjectId } from '../lib/entryPaths'
+import NotFound from './NotFound.jsx'
 
 // v0.17.0 — roadmap item #16. The max length the backend enforces
 // (Pydantic body bound + `invites.CUSTOM_MESSAGE_MAX_LENGTH`); kept
@@ -82,6 +83,7 @@ export default function Admin({ viewer }) {
           {isSiteOwner && <Route path="retired" element={<RetiredTab />} />}
           <Route path="audit" element={<AuditTab />} />
           <Route path="permissions" element={<PermissionsTab />} />
+          <Route path="*" element={<NotFound message="That admin page doesn't exist." />} />
         </Routes>
       </div>
     </div>
diff --git a/frontend/src/components/NotFound.jsx b/frontend/src/components/NotFound.jsx
new file mode 100644
index 0000000..7a3c239
--- /dev/null
+++ b/frontend/src/components/NotFound.jsx
@@ -0,0 +1,21 @@
+// Client-side 404 for routes that don't match. An SPA serves index.html
+// for every non-API path (HTTP 200), so an unknown URL would otherwise
+// render blank — or, worse, silently bounce home. This surfaces a clear
+// "page not found" instead. Used by the top-level router and by the
+// nested /admin, /p/:projectId, and /docs route groups so an invalid
+// subpath (e.g. /admin/users/allowlist/graduation) lands here too.
+import { Link } from 'react-router-dom'
+
+export default function NotFound({ message }) {
+  return (
+    <div className="not-found" role="alert">
+      <h1 className="not-found-code">404</h1>
+      <p className="not-found-message">
+        {message || "We couldn't find that page."}
+      </p>
+      <p className="not-found-actions">
+        <Link to="/">Return to the catalog</Link>
+      </p>
+    </div>
+  )
+}