| Named By: | Barnum Brown & Erich Maren Schlaikjer in 1943 |
| Time Period: | Late Cretaceous, 70 Ma |
| Location: | USA |
| Size: | Uncertain due to lack of remains, but comparison to more complete genera has yielded estimates of around 4.5 meters long |
| Diet: | Herbivore/Omnivore |
| Fossil(s): | Skull remains |
| Classification: | | Chordata | Reptilia | Dinosauria | Ornithischia | Pachycephalosauridae | Pachycephalosaurinae | Pachycephalosaurini | |
| Also known as: | | Tylosteus | Troodon wyomingensis | Dracorex | Stygimoloch | |
Pachycephalosaurus (meaning "thick-headed lizard," from Greek pachys-/pakhus- "thick", kephale/kephale "head" and sauros/sauros "lizard") is a genus of pachycephalosaurid dinosaurs. The type species, P. wyomingensis, is the only known species. 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 creature which is primarily known from a single skull and a few extremely thick skull roofs, though more complete fossils have been found in recent years. Pachycephalosaurus was one of the last non-avian dinosaurs before the Cretaceous-Paleogene extinction event. Another dinosaur, Tylosteus of western North America, has been synonymized with Pachycephalosaurus.
Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore with an extremely thick skull roof. It possessed long hindlimbs and small forelimbs. Pachycephalosaurus is the largest known pachycephalosaur.
The thick skull domes of Pachycephalosaurus and related genera gave rise to the hypothesis that pachycephalosaurs used their skulls in intra-species combat. This hypothesis has been disputed in recent years.