Chasmosaurus (/ˌkæzmoʊˈsɔːrəs/ KAZ-moh-SOR-əs) is a genus of ceratopsid dinosaur from the Late Cretaceous Period in North America. Its given name means 'opening lizard', referring to the large openings (fenestrae) in its frill (Greek chasma, meaning 'opening', 'hollow', or 'gulf'; and sauros, meaning 'lizard'). With a length of 4.3–4.8 metres (14.1–15.7 ft) and a weight of 1.5–2 tonnes (1.7–2.2 short tons)—or anywhere from 2,200 to nearly 5,000 lbs—Chasmosaurus was of a slightly smaller to "average" size, especially when compared to larger ceratopsids (such as Triceratops, which were about the size of an African bush elephant).

It was initially to be called Protorosaurus, but this name had been previously published for another animal. All of the excavated specimens of Chasmosaurus were collected at the Dinosaur Park Formation, Dinosaur Provincial Park, Alberta, Canada. For many years, Chasmosaurus russelli was considered a second species. However, in 2026 its type specimen was given the separate genus Cryptarcus, with other prior "C. russelli" specimens being of uncertain taxonomy.

In 1898, at Berry Creek, Alberta, Lawrence Morris Lambe of the Geological Survey of Canada made the first discovery of Chasmosaurus remains; holotype NMC 491, a parietal bone that was part of a neck frill. Although recognizing that his find represented a new species, Lambe thought this could be placed in a previously known short-frilled ceratopsian genus: Monoclonius. He erected the new species Monoclonius belli to describe his findings. The specific name honoured collector Walter Bell.

However, in 1913, Charles Hazelius Sternberg and his sons found several complete "M. belli" skulls in the middle Dinosaur Park Formation of Alberta, Canada. Based on these finds, Lambe (1914) erected Protorosaurus ("before Torosaurus"), but that name was preoccupied by the Permian reptile Protorosaurus, so he subsequently created the replacement name Chasmosaurus in February 1914. The name Chasmosaurus is derived from Greek χάσμα, khasma, "opening" or "divide" and refers to the very large parietal fenestrae in the skull frill. Lambe now also assigned a paratype, specimen NMC 2245 found by the Sternbergs in 1913 and consisting of a largely complete skeleton, including skin impressions. Apart from the holotype and paratype several additional specimens of C. belli are known. These include AMNH 5422, ROM 843 (earlier ROM 5499) and NHMUK R4948, all (partial) skeletons with skull.

Since that date, more remains, including skulls, have been found that have been referred to Chasmosaurus, and several additional species have been named within the genus. Today several of these are considered to only reflect a morphological variation among the known sample of Chasmosaurus belli skulls.

In addition to Monoclonius belli, later Chasmosaurus, Lambe's 1902 study named Monoclonius canadensis. This species would later be given the distinct genus Eoceratops by Lambe in 1915. In 1990, Thomas Lehmen would refer to this the genus Chasmosaurus, forming the binomial Chasmosaurus canadensis. It was supposedly distinguished by its longer postorbital horns than other species Similarly, in 1933 Barnum Brown named Chasmosaurus kaiseni, honouring Peter Kaisen and based on skull AMNH 5401, also distingished by its very long brow horns. A 2016 analysis of Chasmosaurus specimens by James Campbell and colleagues found that postorbital horn size was too variable to be considered a distinguishing trait, and due to lacking the back of the parietal bone (forming the back of the frill), both species were considered indeterminate beyond the genus Chasmosaurus.

Richard Swann Lull named an unusual, short-muzzled skull, specimen ROM 839 (earlier ROM 5436) collected in 1926, as Chasmosaurus brevirostris in 1933, with the name referring to its short snout. This was seen as a junior synonym of C. belli by Lehman, whereas Campbell and colleagues considered it an indeterminate specimen only referrable to the genus Chasmosaurus.

The species Chasmosaurus irvinensis was named in 2001, stemming from the uppermost beds of the Dinosaur Park Formation. It is distinguished by its unembayed parietal bar and the downwardly curled ornamentation along the back of its frill. This species was given its own genus, Vagaceratops, in 2010. However, a 2019 study by Campbell and colleagues returned Vagaceratops back to Chasmosaurus due to observing that some putative specimens of C. belli show transitional anatomy towards that of C. irvinesis. These transitional specimens were removed from C. belli, and merely considered indeterminate Chasmosaurus. These include the specimens YPM 2016 and AMNH 5402. A 2020 study by Denver Fowler and Elizabeth Freedman Fowler agreed with the evolutionary link between Chasmosaurus and Vagaceratops, but considered the latter an even closer relative and possible ancestor of Kosmoceratops. Consequently, they suggested it should be maintained as a distinct genus from Chasmosaurus, as its placement would probably remain unstable until chasmosaurine relationships are better understood. They also noted that CMN 2245, another former C. belli specimen, represents an additional seemingly transitional specimen between C. belli and Vagaceratops.