diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1284b51429..372dd7cf9a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 ENHANCEMENTS:
 * Specify default_outbound_access_enabled = false setting for all subnets ([#4757](https://github.com/microsoft/AzureTRE/pull/4757))
 * Pin all GitHub Actions workflow steps to full commit SHAs to prevent supply chain attacks plus update to latest releases ([#4886](https://github.com/microsoft/AzureTRE/pull/4886))
+* Add a default UI 404 page for invalid URLs, including a link back to the home page ([#4908](https://github.com/microsoft/AzureTRE/pull/4908))
 
 ## (0.28.0) (March 2, 2026)
 **BREAKING CHANGES**
diff --git a/ui/app/src/components/root/RootLayout.test.tsx b/ui/app/src/components/root/RootLayout.test.tsx
new file mode 100644
index 0000000000..0f76daf73b
--- /dev/null
+++ b/ui/app/src/components/root/RootLayout.test.tsx
@@ -0,0 +1,69 @@
+import React from "react";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { render, screen, waitFor } from "../../test-utils";
+import { RootLayout } from "./RootLayout";
+
+const mockApiCall = vi.fn();
+
+vi.mock("../../hooks/useAuthApiCall", () => ({
+  useAuthApiCall: () => mockApiCall,
+  HttpMethod: { Get: "GET" },
+  ResultType: { JSON: "json" },
+}));
+
+vi.mock("./RootDashboard", () => ({
+  RootDashboard: () => <div>Root Dashboard</div>,
+}));
+
+vi.mock("./LeftNav", () => ({
+  LeftNav: () => <div>Left Nav</div>,
+}));
+
+vi.mock("../shared/RequestsList", () => ({
+  RequestsList: () => <div>Requests List</div>,
+}));
+
+vi.mock("../shared/SharedServices", () => ({
+  SharedServices: () => <div>Shared Services</div>,
+}));
+
+vi.mock("../shared/SharedServiceItem", () => ({
+  SharedServiceItem: () => <div>Shared Service Item</div>,
+}));
+
+vi.mock("../shared/SecuredByRole", () => ({
+  SecuredByRole: ({ element }: { element: React.ReactElement }) => element,
+}));
+
+describe("RootLayout routes", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockApiCall.mockResolvedValue({ workspaces: [] });
+  });
+
+  it("renders root dashboard on home route", async () => {
+    render(<RootLayout />, {
+      children: <></>,
+      initialEntries: ["/"],
+      appRolesContext: { roles: [], setAppRoles: vi.fn() },
+    });
+
+    await waitFor(() => {
+      expect(screen.getByText("Root Dashboard")).toBeInTheDocument();
+    });
+  });
+
+  it("renders a 404 page for an unknown route", async () => {
+    render(<RootLayout />, {
+      children: <></>,
+      initialEntries: ["/does-not-exist"],
+      appRolesContext: { roles: [], setAppRoles: vi.fn() },
+    });
+
+    await waitFor(() => {
+      expect(screen.getByText("404 - Page not found")).toBeInTheDocument();
+    });
+
+    expect(screen.getByRole("link", { name: "Go to home page" })).toHaveAttribute("href", "/");
+  });
+});
diff --git a/ui/app/src/components/root/RootLayout.tsx b/ui/app/src/components/root/RootLayout.tsx
index ab842b3672..1ded601718 100644
--- a/ui/app/src/components/root/RootLayout.tsx
+++ b/ui/app/src/components/root/RootLayout.tsx
@@ -22,6 +22,7 @@ import { ExceptionLayout } from "../shared/ExceptionLayout";
 import { AppRolesContext } from "../../contexts/AppRolesContext";
 import { CostsContext } from "../../contexts/CostsContext";
 import config from "../../config.json";
+import { NotFoundLayout } from "../shared/NotFoundLayout";
 
 export const RootLayout: React.FunctionComponent = () => {
   const [workspaces, setWorkspaces] = useState([] as Array<Workspace>);
@@ -190,6 +191,7 @@ export const RootLayout: React.FunctionComponent = () => {
                           />
                         }
                       />
+                      <Route path="*" element={<NotFoundLayout />} />
                     </Routes>
                   }
                 />
@@ -199,9 +201,11 @@ export const RootLayout: React.FunctionComponent = () => {
                     <Routes>
                       <Route path="/" />
                       <Route path="airlock/*" element={<RequestsList />} />
+                      <Route path="*" element={<NotFoundLayout />} />
                     </Routes>
                   }
                 />
+                <Route path="*" element={<NotFoundLayout />} />
               </Routes>
             </Stack.Item>
           </Stack>
diff --git a/ui/app/src/components/shared/NotFoundLayout.tsx b/ui/app/src/components/shared/NotFoundLayout.tsx
new file mode 100644
index 0000000000..84b57ff944
--- /dev/null
+++ b/ui/app/src/components/shared/NotFoundLayout.tsx
@@ -0,0 +1,15 @@
+import { Link, MessageBar, MessageBarType } from "@fluentui/react";
+import React from "react";
+import { Link as RouterLink } from "react-router-dom";
+
+export const NotFoundLayout: React.FunctionComponent = () => {
+  return (
+    <MessageBar messageBarType={MessageBarType.error} isMultiline={true}>
+      <h2>404 - Page not found</h2>
+      <p>The page you are looking for does not exist.</p>
+      <Link as={RouterLink} to="/">
+        Go to home page
+      </Link>
+    </MessageBar>
+  );
+};
diff --git a/ui/app/src/components/workspaces/WorkspaceProvider.tsx b/ui/app/src/components/workspaces/WorkspaceProvider.tsx
index 076f64f773..acd94f7ceb 100644
--- a/ui/app/src/components/workspaces/WorkspaceProvider.tsx
+++ b/ui/app/src/components/workspaces/WorkspaceProvider.tsx
@@ -32,6 +32,7 @@ import { LoadingState } from "../../models/loadingState";
 import { ExceptionLayout } from "../shared/ExceptionLayout";
 import { AppRolesContext } from "../../contexts/AppRolesContext";
 import { RoleName, WorkspaceRoleName } from "../../models/roleNames";
+import { NotFoundLayout } from "../shared/NotFoundLayout";
 
 export const WorkspaceProvider: React.FunctionComponent = () => {
   const apiCall = useAuthApiCall();
@@ -335,9 +336,10 @@ export const WorkspaceProvider: React.FunctionComponent = () => {
                           path="shared-services/:sharedServiceId/*"
                           element={<SharedServiceItem readonly={true} />}
                         />
-                        <Route path="requests/*" element={<Airlock />} />
-                      </>)}
+                         <Route path="requests/*" element={<Airlock />} />
+                       </>)}
                     <Route path="users/*" element={<WorkspaceUsers />} />
+                    <Route path="*" element={<NotFoundLayout />} />
                   </Routes>
                 </Stack.Item>
               </Stack>