pt5m is a 0.5 m robotic telescope located on the roof of the 4.2 m William Herschel Telescope (WHT) building, at the Roque de los Muchachos Observatory, La Palma. Using a five-position filter wheel and CCD detector, and bespoke control software, pt5m provides a high-quality robotic observing facility. The telescope first began robotic observing in 2012, and is now contributing to transient follow-up and time-resolved astronomical studies. In this paper, we present the scientific motivation behind pt5m, as well as the specifications and unique features of the facility. We also present an example of the science we have performed with pt5m, where we measure the radius of the transiting exoplanet WASP-33b. We find a planetary radius of 1.603 ± 0.014RJ.