Triceratops (/traɪˈsɛrətɒps/ try-SERR-ə-tops; lit. 'three-horned face') is a genus of chasmosaurine ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous period, about 68 to 66 million years ago on the island continent of Laramidia, now forming western North America. It was one of the last-known non-avian dinosaurs and lived until the Cretaceous–Paleogene extinction event 66 million years ago. The name Triceratops, which means 'three-horned face', is derived from the Ancient Greek words τρί- (trí-), meaning "three", κέρας (kéras), meaning "horn", and ὤψ (ṓps), meaning "face".

Bearing a large bony frill, three horns on the skull, and a large, four-legged body, exhibiting convergent evolution with rhinoceroses, Triceratops is one of the most recognizable of all dinosaurs and the best-known ceratopsian. It was also one of the largest, measuring around 8–9 m (26–30 ft) long and weighing up to 6–10 t (5.9–9.8 long tons; 6.6–11.0 short tons). It shared the landscape with and was most likely preyed upon by Tyrannosaurus. The functions of the frills and three distinctive facial horns on its head have inspired countless debates. Traditionally, these have been viewed as defensive weapons against predators. More recent interpretations find it probable that these features were primarily used in species identification, courtship, and dominance display, much like the antlers and horns of modern ungulates.

Triceratops was traditionally placed within the "short-frilled" ceratopsids, but modern cladistic studies show it to be a member of Chasmosaurinae, which usually have long frills. Two species, T. horridus and T. prorsus, are considered valid today. Seventeen different species, however, have been named throughout history. Research published in 2010 argued that the contemporaneous Torosaurus, a ceratopsid long regarded as a separate genus, represents Triceratops in its mature form. This view has been highly disputed, with some morphometric research and the apparent existence of authentic Torosaurus subadults favoring the distinctness of the two genera, according to newer studies.

Triceratops has been documented by numerous remains collected since the genus was first described in 1889 by American paleontologist Othniel Charles Marsh. Specimens representing life stages from hatchling to adult have been found. As the archetypal ceratopsian, Triceratops is one of the most beloved, popular dinosaurs and has been featured in numerous films, postage stamps, and many other media types.

The first named fossil specimen now attributed to Triceratops is a pair of brow horns attached to a skull roof that were found by George Lyman Cannon near Denver, Colorado, in the spring of 1887. This specimen was sent to Othniel Charles Marsh, who believed that the formation from which it came dated from the Pliocene and that the bones belonged to a particularly large and unusual bison, which he named Bison alticornis. He realized that there were horned dinosaurs by the next year, which saw his publication of the genus Ceratops from fragmentary remains, but he still believed B. alticornis to be a Pliocene mammal. It took a third and much more complete skull to fully change his mind.^{[citation needed]}

Although not confidently assignable, fossils possibly belonging to Triceratops were described as two taxa, Agathaumas sylvestris and Polyonax mortuarius, in 1872 and 1874, respectively, by Marsh's archrival Edward Drinker Cope. Agathaumas was named based on a pelvis, several vertebrae, and a few ribs collected by Fielding Bradford Meek and Henry Martyn Bannister near the Green River of southeastern Wyoming from layers coming from the Maastrichtian Lance Formation. Due to the fragmentary nature of the remains, it can only confidently be assigned to Ceratopsidae. Polyonax mortuarius was collected by Cope himself in 1873 from northeastern Colorado, possibly coming from the Maastrichtian Denver Formation. The fossils only consisted of fragmentary horn cores, 3 dorsal vertebrae, and fragmentary limb elements. Polyonax has the same issue as Agathaumas, with the fragmentary remains non-assignable beyond Ceratopsidae.

The Triceratops holotype, YPM 1820, was collected in 1888 from the Lance Formation of Wyoming by fossil hunter John Bell Hatcher, but Marsh initially described this specimen as another species of Ceratops. Cowboy Edmund B. Wilson had been startled by the sight of a monstrous skull poking out of the side of a ravine. He tried to recover it by throwing a lasso around one of the horns. When it broke off, the skull tumbling to the bottom of the cleft, Wilson brought the horn to his boss. His boss was rancher and avid fossil collector Charles Arthur Guernsey, who just so happened to show it to Hatcher. Marsh subsequently ordered Hatcher to locate and salvage the skull. The holotype was first named Ceratops horridus. When further preparation uncovered the third nose horn, Marsh changed his mind and gave the piece the new generic name Triceratops (lit. 'three horn face'), accepting his Bison alticornis as another species of Ceratops. It would, however, later be added to Triceratops.

After Triceratops was described, between 1889 and 1891, Hatcher collected another thirty-one of its skulls with great effort. The first species had been named T. horridus by Marsh. Its specific name was derived from the Latin word horridus meaning "rough" or "rugose", perhaps referring to the type specimen's rough texture, later identified as an aged individual. The additional skulls varied to a lesser or greater degree from the original holotype. This variation is unsurprising, given that Triceratops skulls are large three-dimensional objects from individuals of different ages and both sexes that which were subjected to different amounts and directions of pressure during fossilization.