Pachycephalosaurus wyomingensis ("thick headed lizard" ) is a genus of pachycephalosaurid dinosaur. It lived during the Late Cretaceous Period (about 76 to 65 million years ago) of what is now North America. Remains have been excavated in Montana, South Dakota, and Wyoming. It was an herbivorous or maybe omnivorous creature.
Like other pachycephalosaurids, Pachycephalosaurus was a bipedal omnivore with an extremely thick skull roof. It is the largest known pachycephalosaur, measuring about 5 metres (15 to 16 ft) in length.