diff --git a/src/apps/platform/src/platform.routes.tsx b/src/apps/platform/src/platform.routes.tsx
index a39f1f82e..5a8f9d691 100644
--- a/src/apps/platform/src/platform.routes.tsx
+++ b/src/apps/platform/src/platform.routes.tsx
@@ -25,6 +25,11 @@ const Home: LazyLoadedComponent = lazyLoad(
     'HomePage',
 )
 
+const NotFound: LazyLoadedComponent = lazyLoad(
+    () => import('./routes/not-found'),
+    'NotFoundPage',
+)
+
 const homeRoutes: ReadonlyArray<PlatformRoute> = [
     {
         element: <Home />,
@@ -33,6 +38,17 @@ const homeRoutes: ReadonlyArray<PlatformRoute> = [
     },
 ]
 
+// Catch-all for paths Platform UI is served for but does not own a route for,
+// eg. `/opportunities` on the Topcoder apex host. React-router ranks the `*`
+// path last, so this never shadows a route declared above.
+const notFoundRoutes: ReadonlyArray<PlatformRoute> = [
+    {
+        element: <NotFound />,
+        id: 'Not found page',
+        route: '*',
+    },
+]
+
 export const platformRoutes: Array<PlatformRoute> = [
     // NOTE: Order matters here bc the active tool
     // is determined by finding the first route
@@ -57,4 +73,5 @@ export const platformRoutes: Array<PlatformRoute> = [
     ...adminRoutes,
     ...reportsRoutes,
     ...customerPortalRoutes,
+    ...notFoundRoutes,
 ]
diff --git a/src/apps/platform/src/routes/not-found/NotFound.module.scss b/src/apps/platform/src/routes/not-found/NotFound.module.scss
new file mode 100644
index 000000000..6fab0c29a
--- /dev/null
+++ b/src/apps/platform/src/routes/not-found/NotFound.module.scss
@@ -0,0 +1,31 @@
+@import "@libs/ui/styles/includes";
+
+.container {
+    min-height: 60vh;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}
+
+.content {
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    text-align: center;
+    gap: $sp-6;
+    padding: $sp-15 $sp-8;
+    color: $black-100;
+
+    svg {
+        color: $black-60;
+    }
+}
+
+.title {
+    @include font-barlow;
+    font-weight: 600;
+    font-size: 28px;
+    line-height: 34px;
+    margin: 0;
+}
diff --git a/src/apps/platform/src/routes/not-found/NotFound.spec.tsx b/src/apps/platform/src/routes/not-found/NotFound.spec.tsx
new file mode 100644
index 000000000..f2cb983d1
--- /dev/null
+++ b/src/apps/platform/src/routes/not-found/NotFound.spec.tsx
@@ -0,0 +1,53 @@
+/* eslint-disable import/no-extraneous-dependencies, ordered-imports/ordered-imports */
+import '@testing-library/jest-dom'
+import type { PropsWithChildren } from 'react'
+import { render, screen } from '@testing-library/react'
+import { MemoryRouter, Route, Routes } from 'react-router-dom'
+
+import NotFoundPage from './NotFound'
+
+jest.mock('~/libs/ui', () => ({
+    ContentLayout: (props: PropsWithChildren): JSX.Element => <div>{props.children}</div>,
+    IconOutline: {
+        ExclamationCircleIcon: (): JSX.Element => <svg />,
+    },
+    LinkButton: (props: PropsWithChildren<{ to: string }>): JSX.Element => (
+        <a href={props.to}>{props.children}</a>
+    ),
+    PageTitle: (): JSX.Element => <></>,
+}), { virtual: true })
+
+// `getRouteElement` renders a childless `PlatformRoute` as `<Route path={route.route} />`,
+// so these paths mirror the platform `homeRoutes` and `notFoundRoutes` entries.
+function renderPlatformRoutes(pathname: string): void {
+    render(
+        <MemoryRouter initialEntries={[pathname]}>
+            <Routes>
+                <Route element={<div>home page</div>} path='' />
+                <Route element={<NotFoundPage />} path='*' />
+            </Routes>
+        </MemoryRouter>,
+    )
+}
+
+describe('NotFoundPage', () => {
+    it('renders a message instead of a blank page for an unmatched path', () => {
+        renderPlatformRoutes('/opportunities')
+
+        expect(screen.getByRole('alert'))
+            .toBeInTheDocument()
+        expect(screen.getByText('We were unable to find that page'))
+            .toBeInTheDocument()
+        expect(screen.getByText('Go to the home page'))
+            .toHaveAttribute('href', '/')
+    })
+
+    it('does not shadow a route the platform does own', () => {
+        renderPlatformRoutes('/')
+
+        expect(screen.getByText('home page'))
+            .toBeInTheDocument()
+        expect(screen.queryByRole('alert'))
+            .not.toBeInTheDocument()
+    })
+})
diff --git a/src/apps/platform/src/routes/not-found/NotFound.tsx b/src/apps/platform/src/routes/not-found/NotFound.tsx
new file mode 100644
index 000000000..790e6bd29
--- /dev/null
+++ b/src/apps/platform/src/routes/not-found/NotFound.tsx
@@ -0,0 +1,35 @@
+import { FC } from 'react'
+
+import {
+    ContentLayout,
+    IconOutline,
+    LinkButton,
+    PageTitle,
+} from '~/libs/ui'
+
+import styles from './NotFound.module.scss'
+
+/**
+ * Fallback page for a path Platform UI is served for but has no route for.
+ *
+ * Without it react-router matches nothing, the router renders an empty
+ * container inside the app shell, and the user is shown a blank page.
+ */
+const NotFoundPage: FC<{}> = () => (
+    <ContentLayout outerClass={styles.container}>
+        <PageTitle>Page Not Found | Topcoder</PageTitle>
+
+        <div className={styles.content} role='alert'>
+            <IconOutline.ExclamationCircleIcon className='icon-xxxl' />
+            <h2 className={styles.title}>We were unable to find that page</h2>
+            <p className='body-main'>
+                The page you requested does not exist or is not available yet.
+            </p>
+            <LinkButton primary size='lg' to='/'>
+                Go to the home page
+            </LinkButton>
+        </div>
+    </ContentLayout>
+)
+
+export default NotFoundPage
diff --git a/src/apps/platform/src/routes/not-found/index.ts b/src/apps/platform/src/routes/not-found/index.ts
new file mode 100644
index 000000000..6668cd4c6
--- /dev/null
+++ b/src/apps/platform/src/routes/not-found/index.ts
@@ -0,0 +1 @@
+export { default as NotFoundPage } from './NotFound'