Centrosaurus (/ˌsɛntroʊˈsɔːrəs/ SEN-troh-SOR-əs; lit. 'pointed lizard') is a genus of centrosaurine ceratopsid dinosaur from the Campanian age of the Late Cretaceous of Canada. Its remains have been found in the Dinosaur Park Formation, dating from 76.5 to 75.5 million years ago. The type and only species seen as valid today is Centrosaurus apertus.

The first Centrosaurus remains were discovered and named by paleontologist Lawrence Lambe in strata along the Red Deer River in Alberta. The name Centrosaurus means "pointed lizard" (from Greek kentron, κέντρον, "point" and sauros, σαῦρος, "lizard") and refers to, the series of small hornlets placed along the margin of their frills, not the nasal horns (which were unknown when the dinosaur was named). The genus is not to be confused with the stegosaur Kentrosaurus, the name of which is derived from the same Greek word.

Later, vast bonebeds of Centrosaurus were found in Dinosaur Provincial Park, also in Alberta. Some of these beds extend for hundreds of meters and contain thousands of individuals of all ages and all levels of completion. Scientists have speculated that the high density and number of individuals would be explained if they had perished while trying to cross a flooded river. A discovery of thousands of Centrosaurus fossils near the town of Hilda, Alberta, is believed to be the largest bed of dinosaur bones ever discovered. The area is now known as the Hilda mega-bonebed.

Because of the variation between species and even individual specimens of centrosaurines, there has been much debate over which genera and species are valid, particularly whether Centrosaurus and/or Monoclonius are valid genera, undiagnosable, or possibly members of the opposite sex. In 1996, Peter Dodson found enough variation between Centrosaurus, Styracosaurus, and Monoclonius to warrant separate genera and that Styracosaurus resembled Centrosaurus more closely than either resembled Monoclonius.

Dodson believed one species of Monoclonius, M. nasicornus, may actually have been a female Styracosaurus. His assessments have been partially followed, with other researchers not accepting Monoclonius nasicornus as a female Styracosaurus or Monoclonius as a valid genus. While sexual dimorphism has been proposed for a more basal ceratopsian, Protoceratops, there is no firm evidence for sexual dimorphism in any ceratopsid. Others have synonymized C. nasicornus with C. apertus, or considered it a separate Centrosaurus species: Centrosaurus nasicornus. It has also been suggested as the direct ancestor of Styracosaurus albertensis. A 2014 study of changes during growth in Centrosaurus concluded that C. nasicornus is a junior synonym of C. apertus, representing a middle growth stage.

The species C. brinkmani, described in 2005, was moved to the new genus Coronosaurus in 2012.

Centrosaurus were large dinosaurs, although not as large as some of their relatives, reaching 5–5.5 metres (16–18 ft) long and 2–2.5 metric tons (2.2–2.8 short tons) in body mass situated atop stocky limbs. Like other centrosaurines, Centrosaurus bore a single large horn over their noses. These horns curved forwards or backwards depending on the specimen. Skull ornamentation was reduced as animals aged. The frill was relatively short compared to the total skull length and could grow to over half a meter (68.8 cm) long in the oldest and largest adults.

Centrosaurus is distinguished by having two large hornlets which hook forwards over the frill. A pair of small upwards directed horns is also found over the eyes. The frills of Centrosaurus were moderately long, with fairly large fenestrae and small hornlets along the outer edges.