Pachycephalosaurus (pronounced /ˌpækiˌsɛfələˈsɔrəs/, meaning "thick headed lizard", from Greek pachy-/παχυ- "thick", cephale/κεφαλη "head" and saurus/σαυρος "lizard") is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period (Maastrichtian stage) of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or omnivorous creature which is only known from a single skull and a few extremely thick skull roofs. This dinosaur is monotypic, meaning the type species, P. wyomingensis, is the only known species. Pachycephalosaurus was one of the last non-avian dinosaurs before the K–T extinction event. Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus.
Like other pachycephalosaurids, Pachycephalosaurus was a bipedal omnivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. Pachycephalosaurus is the largest known pachycephalosaur, measuring nearly 4.5 to 5 metres (15 to 16 ft) in length and up to 2,000 kilograms (4,400 lb) in weight.
The thick skull domes of Pachycephalosaurus and related genera gave rise to the theory that pachycephalosaurs used their skulls in intraspecific combat. This theory has been discredited in recent years.