EDIT: Updated to reflect React 16

Custom attributes are supported natively in React 16. This means that adding a custom attribute to an element is now as simple as adding it to a render function, like so:

render() {

  return (

    <div custom-attribute=some-value />

  );

}

For more:

https://reactjs.org/blog/2017/09/26/react-v16.0.html#support-for-custom-dom-attributes

https://facebook.github.io/react/blog/2017/09/08/dom-attributes-in-react-16.html

Previous answer (React 15 and earlier)

Custom attributes are currently not supported. See this open issue for more info: https://github.com/facebook/react/issues/140

As a workaround, you can do something like this in componentDidMount:

componentDidMount: function() {

  var element = ReactDOM.findDOMNode(this.refs.test);

  element.setAttribute('custom-attribute', 'some value');

}

See https://jsfiddle.net/peterjmag/kysymow0/ for a working example. (Inspired by syranide's suggestion in this comment.)