Diabloceratops /daɪˌæbloʊˈsɛrətɒps/ [dee-ab-lo-ser-a-tops] is an outstanding and bizarre genus of centrosaurine ceratopsian that lived approximately 79 million years ago during the latter part of the Cretaceous Period in what is now Utah, in the United States. Diabloceratops was a medium sized, moderately-built, ground-dwelling, quadrupedal herbivore, that could grow up to an estimated 5.5 m (18.0 ft) long. At the time of its discovery, it was the oldest known ceratopsid, and first centrosaurine known from latitudes south of the US state of Montana.
Diabloceratops was built like a typical ceratopsian in that it had a large neck frill made of bone. It had a small horn on the nose, perhaps a second horn in front of that, and a pair of relatively small horns above the eyes. The skull is deeper and shorter than that of any other centrosaurines. Upon the frill it also had a pair of very long spikes as in Einiosaurus and Styracosaurus. It being one of the earliest centrosaurine ceratopsids, Kirkland noted a character Diabloceratops shared with the more "primitive" protoceratopsid forms. Both possess an accessory opening in the skull that would become much reduced or disappear in later, more advanced ceratopsids. Kirkland saw this as an indication that the earlier species were not together included in some single natural group but instead presented a gradual sequence of ever more derived forms, increasingly closer related to the Ceratopsidae.