Originally described by O. C. Marsh in 1879 as Camptonotus, or "bent back", it was renamed Camptosaurus in 1885 because the original name was already in use for a cricket. In 1879, Marsh named C. dispar (type species of the genus) and C. amplus for material he received from his collectors at Quarry 13 near Como Bluff, Wyoming. Throughout the 1880-1890s, he continued to receive specimens from Quarry 13 and named two additional species: C. medius and C. nanus based in part on size. Charles W. Gilmore named two additional species, C. browni and C. depressus in his 1909 redescription of the Marsh specimens.
Then in 1980, Peter Galton and H.P. Powell in their redescription of C. prestwichi (see following), considered C. nanus, C. medius and C. browni to be different growth stages or different gender of the larger C. dispar, and therefore only C. dispar was the valid species. They also considered the skull referred to C. amplus by Marsh and by Gilmore to belong to C. dispar, as well. Gilmore had used this skull to describe the skull of Camptosaurus, but the specimen was recently shown by Brill and Carpenter not to belong to Camptosaurus and put it into its own genus and species, Theiophytalia kerri.
A second species of Camptosaurus was recently named by Carpenter and Wilson for specimens from Dinosaur National monument. It differs from C. dispar in the lower jaw, shorter neck vertebrae, and straighter ischium ending in a small "foot" among other features. While Marsh was describing Camptosaurus species in North America, numerous species from Europe were also referred to the genus in the late 1800s and early 1900s, as well: C. inkeyi, C. hoggii, C. leedsi, C. prestwichii, and C. valdensis. C. inkeyi and C. leedsi consist of fragmentary material and species are no longer considered valid (nomen dubium). C. valdensis was moved to Valdosaurus canaliculatus by Galton in 1977. C. hoggii was originally named Iguanodon hoggii by Richard Owen in 1874 and was moved to Camptosaurus by Norman and Barrett in 2002.
The remaining European species Camptosaurus prestwichii was recovered from Chawley Brick Pits, Cumnor Hurst in Oxfordshire in England. The fossil was found when a tramway was driven into the side of a hill. It was described by Hulke in 1880 as Iguanodon prestwichii, and then placed in its own genus Cumnoria by Seeley in 1888. It was moved to Camptosaurus by Lydekker in 1889 where it remains today. The holotype skeleton (see picture) remains the best skeleton of Camptosaurus from Europe.