Abstract Intensity interferometry is well established with visible light but has never been demonstrated with X-radiation. We propose to measure the transverse coherence of an X-ray beam using the method of Hanbury Brown and Twiss. The X-ray interferometer consists of an array of slits, a grazing incidence reflective beam splitter, a pair of fast multichannel plate detectors and a broadband, low-noise correlator circuit. The NSLS X1 or X13 soft X-ray undulator will supply the partially coherent X-rays. We are developing this technique to characterize the coherence properties of X-ray beams from high brilliance insertion devices at third-generation synchrotron light facilities such as the Advanced Photon Source and the Advanced Light Source.