Pachyrhinosaurus (“thick-nosed reptile") is an ornithischian (bird-hipped) dinosaur from the Late Cretaceous time of North America. The first examples were exposed by Charles M. Sternberg in Alberta, Canada, in 1946, and named in 1950. 12 partial skulls and a large collection of fossils have been established in total in Alberta and Alaska. A great number were not available for study until the 1980s, resulting in a comparatively recent increase of interest in the Pachyrhinosaurus.

Instead of horns, the skull bears massive, flattened bosses, the largest being over the nose. These were almost certainly used in butting and shoving matches, as in musk oxen. A single pair of horns grew from the frill and comprehensive upwards. It appears that that both of the shape and size of the frill was highly individualized, reliant on gender and perhaps other factors. Pachyrhinosaurus is most closely connected to Achelousaurus.

Pachyrhinosaurus was 18 to 23 feet (5.5 to seven meters) lengthy and about six feet (two meters) high. It weighed about two tons.

Like other ceratopsid dinosaurs, Pachyrhinosaurus was probable a herd animal, traveling in large groups and caring for its young. It was herbivorous and obsessed strong cheek teeth to help it chew tough, fibrous plants. In addition, its beak was jagged. Pachyrhinosaurus could almost certainly also run at speeds of up to 20 miles per hour (32 kilometers per hour), an ability which would advantage the strength of its defenses.

When threatened, it probably emotional at its opponent in much the same way modern rhinoceroses do today. The massive, flattened boss on the end of its nose could absorb the impact while severely injuring whoever received the blunt of their blows. (There is some debate as to whether the boss was in fact an anchor for the sharp horn found on the nose of most of its close relatives.)

According to its Encephalization Quotient, the Pachyrhinosaurus was of middle intelligence as compared to other dinosaurs.