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Achelousaurus ( , less commonly: ) is a genus of herbivorous centrosaurine ceratopsid dinosaurs from the Late Cretaceous Period of what is now North America, dated to 74.2 million years ago. The first fossils of Achelousaurus were dug up in 1987, in the north of Montana, by a team led by Jack Horner. In 1989, more finds were made. In 1995, Achelousaurus got its name, which means "Achelous lizard", when the type species Achelousaurus horneri was named by Scott D. Sampson. This is the only species named in the genus. The discoveries mainly consisted of a small number of skulls. Some of these were of individuals that were not yet fully grown. Parts of the rest of the skeleton have also been found but have not yet been described.

Achelousaurus was an average-sized centrosaurine. It was about six metres long and weighed three tonnes. It walked on all fours. It had a large head with a bony shield at the rear, covering the neck. The shield sported a pair of long spikes, curving towards the outside. Other centrosaurines often had a pair of horns above the eyes and a long horn on the snout. Adult Achelousaurus instead had rough bosses in the same positions. These bumps were covered by a thick layer of keratin. According to some scientists the bosses were used in fights, with the animals butting each other's heads.

Horner in 1987, in slightly older layers discovered another horned dinosaur, Einiosaurus. It too had the long spikes, but not the bosses. It was already known that in a later period Pachyrhinosaurus lived, a species with very large bosses. Horner claimed in 1992 that Achelousaurus was the direct descendant of Einiosaurus and the direct ancestor of Pachyrhinosaurus. Most scientists do not think that Horner has proven his claim. However, later discoveries show that Achelousaurus is closely related to Pachyrhinosaurus in the group Pachyrhinosaurini.