Achelousaurus ( /əˌkiːloʊˈsɔːrəs, ˌækɪˌloʊəˈsɔːrəs/) is a genus of centrosaurine ceratopsid dinosaur that lived during the Late Cretaceous Period of what is now North America, about 77 to 74.8 million years ago. The first fossils of Achelousaurus were collected in Montana in 1987, by a team led by Jack Horner, with more finds made in 1989. In 1994, Achelousaurus horneri was described and named by Scott D. Sampson; the generic name means "Achelous lizard", in reference to the Greek deity Achelous, and the specific name refers to Horner. The genus is known from a few specimens consisting mainly of skull material from individuals, ranging from juveniles to adults.

A large centrosaurine, Achelousaurus supposedly was about 6 m (20 ft) long, with a weight of about 3 t (3.3 short tons). As a ceratopsian, it walked on all fours, had a short tail and a large head with a hooked beak. It had a bony neck-frill at the rear of the skull, which sported a pair of long spikes, which curved towards the outside. Adult Achelousaurus had rough bosses (roundish protuberances) above the eyes and on the snout where other centrosaurines often had horns in the same positions. These bosses were covered by a thick layer of keratin, but their exact shape in life is uncertain. Some researchers hypothesize that the bosses were used in fights, with the animals butting each other's heads, as well as for display.

Within the Ceratopsia, Achelousaurus lies within the clade Pachyrostra (or "thick-snouts"). It has been suggested that it was the direct descendant of the similar genus Einiosaurus (which had spikes but no bosses) and the direct ancestor of Pachyrhinosaurus (which had larger bosses). The first two genera would be transitional forms, evolving through anagenesis from Styracosaurus. There has been debate about this theory, with later discoveries showing that Achelousaurus is closely related to Pachyrhinosaurus in the group Pachyrhinosaurini. Achelousaurus is known from the Two Medicine Formation and lived in the island continent of Laramidia. As a ceratopsian, Achelousaurus would have been a herbivore and it appears to have had a high metabolic rate, though lower than that of modern mammals and birds.

All known Achelousaurus specimens were recovered from the Two Medicine Formation in Glacier County, Montana during excavations conducted by the Museum of the Rockies, which still houses the specimens. The discoveries came about by an accidental chain of events. In the spring of 1985, paleontologist John "Jack" R. Horner was informed that he would no longer be allowed to exploit the Willow Creek site, where he had studied the Maiasaura Egg Mountain nesting colony for six years. Having already made extensive arrangements for a new field season, he was suddenly forced to seek an alternative site. Horner had always been intrigued by the field diaries of Charles Whitney Gilmore who had reported the discovery of dinosaur eggs at Landslide Butte in 1928, but never published on them. In this locality, Gilmore had employed George Fryer Sternberg to excavate skeletons of the horned dinosaurs Brachyceratops and Styracosaurus ovatus. That summer, Horner obtained the permission of the Blackfeet Indian Tribal Council to prospect for fossils on Landslide Butte, which is part of the Blackfeet Indian Reservation; it was the first paleontological investigation there since the 1920s. In August 1985, Horner's associate Bob Makela discovered a rich fossil site on the land of the farmer Ricky Reagan, which was called the Dinosaur Ridge Quarry and contained fossils of horned dinosaurs. On 20 June 1986, Horner and Makela returned to the Blackfeet Indian Reservation and resumed work on the Dinosaur Ridge Quarry, which proved to contain, apart from eggs, more than a dozen skeletons of a horned dinosaur later named Einiosaurus. In August 1986, at a nearby site – the Canyon Bone Bed on the land of Gloria Sundquist, east of the Milk River – Horner's team discovered another Einiosaurus bone bed. Part of the discoveries made on this occasion was an additional horned dinosaur skull, specimen MOR 492, that later would be referred to (i.e., formally assigned to) Rubeosaurus, the genus name in 2010 given to Styracosaurus ovatus.

During the field season of 1987 (early July), volunteer Sidney M. Hostetter located another horned dinosaur skull near the Canyon Bone Bed, specimen MOR 485. By the end of August, it had been secured and was driven on a grain truck to the Museum of the Rockies in Bozeman. On 23 June 1988, another site was discovered in the vicinity – the Blacktail Creek North. In the summer of 1989, graduate student Scott D. Sampson joined the team, wanting to study the function of the frill display structures in horned dinosaurs. At the end of June 1989, Horner, his son Jason and his head preparator Carrie Ancell discovered horned dinosaur specimen MOR 591, a subadult skull and partial postcranial skeleton, near the Blacktail Creek.

It was initially assumed that all the horned dinosaur material recovered by the expeditions could be assigned to a single "styracosaur" species distinct from Styracosaurus albertensis, as the fossils represented a limited geological time period, then estimated at half a million years. Raymond Robert Rogers, who was studying the stratigraphy of the bone beds, referred to it as a Styracosaurus sp. (of undetermined species) in 1989. Styracosaurus ovatus – though sometimes considered an invalid nomen dubium – had already been found in the area by G. F. Sternberg and was an obvious candidate. But also the possibility was taken into account that the finds were of a species new to science. This species was informally named "Styracosaurus makeli" in honor of Bob Makela, who had died in a traffic accident just days before the discovery of specimen MOR 485. In 1990, this name, as an invalid nomen nudum, appeared in a photo caption in a book by Stephen Czerkas.

Horner, an expert on the Hadrosauridae family, had less affinity for other kinds of dinosaurs. In 1987 and 1989, horned dinosaur specialist Peter Dodson was invited to investigate the new ceratopsian finds. In 1990, the fossil material was seen by Dodson as strengthening the case for the validity of a separate Styracosaurus ovatus, to be distinguished from Styracosaurus albertensis.

Meanwhile, Horner had come to a more complex view of the situation. He still thought that the fossil material had been part of a single population but concluded that this had developed over time as a chronospecies evolving into a series of subsequent taxa. In 1992, Horner, David Varricchio, and Mark Goodwin published an article in Nature based on the six-year field study of sediments and dinosaurs from Montana. They proposed that the expeditions had uncovered three "transitional taxa" spanning the gap between the already known Styracosaurus and Pachyrhinosaurus. For the moment, they declined to name these taxa. The oldest form was indicated as "Transitional Taxon A," mainly represented by skull MOR 492. Then came "Taxon B" – the many skeletons of the Dinosaur Ridge Quarry and the Canyon Bone Bed. The youngest was "Taxon C," represented by skull MOR 485 and the horned dinosaur fossils of the Blacktail Creek. In a 1997 book, Horner referred to the three taxa as "centrosaurine 1.", "centrosaurine 2." and "centrosaurine 3.".