Client side "404 not found"

To create a client side "404 not found" fallback all you need to do is make use of a <Route /> component with a non-greedy matching path.

Create your NotFound component, i.e. the page which will display 404 error message:

// components/NotFound.js
import React from 'react';

const NotFound = () =>
    <h3>404 page not found</h3>
    <p>We are sorry but the page you are looking for does not exist.</p>

export default NotFound;

Let's add <NotFound /> component to routes configuration, using * as a value of the path parameter to get a non-greedy matching.

It is important to remember that the 404 route needs to be declared in the very bottom of your routes configuration, so the <Route /> is only mounted if any of the routes' path declared above are not matched:

import React from 'react';
import { render } from 'react-dom';
import { Router, Route, browserHistory } from 'react-router';
import App from './components/App';
import NotFound from './components/NotFound';

const Root = () =>
  <Router history={browserHistory}>
    <Route path="/" component={App}>

      {/* Your other routes here */}

      <Route path="*" component={NotFound} />

render(<Root />, document.getElementById('root'));

See the live example below, try to enter some non valid url to see 404 page:

source code