Camarasaurus (/ˌkæmərəˈsɔːrəs/ KAM-ər-ə-SOR-əs) is a genus of sauropod dinosaur that lived during the Late Jurassic in what is now the United States. Its fossils are primarily known from the Morrison Formation dating to the Kimmeridgian and Tithonian ages of the Jurassic, between 155 and 145 million years ago (mya). It was named by Edward Drinker Cope in 1877; it definitively contains three species, C. supremus, the type species, C. grandis, and C. lentus, while some researchers consider C. lewisi to be in its own genus, Cathetosaurus. The generic name means "chambered lizard", referring to the hollow chambers, known as pleurocoels, in its cervical vertebrae (Greek καμαρα [kamara] meaning 'vaulted chamber', or anything with an arched cover, and σαυρος [sauros] meaning 'lizard'). Hundreds of specimens have been excavated to date, including some nearly complete skeletons. It is among the best understood dinosaurs, with detailed studies of its anatomy, paleobiology, paleoecology, and more.

A medium-large sauropod, C. supremus is estimated to have been 23 m (75 ft) long and to have weighed 42.3 tonnes whereas C. lentus was only around 15 m (49 ft) long. Like other sauropods, it was quadrupedal, with a large body, long neck, and long tail. Its skull was large, tall, and boxy with a huge naris, in contrast to the low skulls of sauropods like Diplodocus and Apatosaurus. Camarasaurus teeth are distinct in their spoon-shaped teeth which bear continuous cutting edges. It had 13-14 teeth in the upper jaws and 13 teeth in the lower jaws, which were rapidly replaced. Camarasaurus was named during the Bone Wars, a scientific rivalry between paleontologists Edward Drinker Cope and Othniel Charles Marsh. Marsh assigned many species to his genus Morosaurus, including M. lentus and M. grandis which are now believed to be species of Camarasaurus.

Camarasaurus is a member of the group Macronaria, however its relationships with its relatives are uncertain and debated. Some paleontologists have grouped Camarasaurus with taxa like Tehuelchesaurus and Lourinhasaurus in the family Camarasauridae, whereas others believe it is the only member of Camarasauridae. Camarasaurus is thought to have been a medium browser, with a feeding height of 2 to 5 m (6 ft 7 in to 16 ft 5 in), and a selective feeder. The robust, large jaws of the genus indicate that it had a stronger bite than sauropods like diplodocids. This would allow for niche partitioning in the Morrison Formation where fossils of Camarasaurus are known, which preserves a menagerie of sauropods. It is among the most common sauropods in the formation and was adapted to many environments based on its wide distribution. Camarasaurus also lived alongside dinosaurs like Stegosaurus, Allosaurus, and Brachiosaurus.

Fossils of Camarasaurus were first discovered in the spring of 1877 by Oramel William Lucas, a school superintendent, in Morrison Formation rocks in Garden Park, Colorado. These fossils were sent to Edward Drinker Cope, a paleontologist based in Philadelphia, who identified it as belonging to a new genus and species, which he named Camarasaurus supremus, meaning "chambered lizard, from the Greek καμαρα (kamara) meaning "vaulted chamber", or anything with an arched cover, and σαυρος (sauros) meaning "lizard"), and from the Latin supremus, "supreme" or "highest". By this time, the Morrison Formation had become the main battleground of the Bone Wars, a fossil-collecting competition between Cope and paleontologist Othniel Charles Marsh. Due to this, at the time many descriptions of taxa, such as that of Camarasaurus, were rushed by Cope and Marsh. Camarasaurus supremus' type specimen (AMNH 5760) consists of: one cervical vertebra, the namesake of the genus, two dorsal vertebrae, and three caudal vertebrae. However, these elements are from differing individuals, making them cotypes. The cervical vertebra has been proposed to be the lectotype of C. supremus, though there has not been a published stance.

Between August 1877 and 1878, Lucas unearthed and sent many more C. supremus fossils, including: two cervicals, 18 dorsals, 55 caudals, 17 left ribs and several right ones, eight chevrons, one right scapula, one right coracoid, two pubes, four ischia, one femur, and one fibula. These elements came from the "Yellow Beds" of Garden Park and were also grouped with AMNH 5760, as Cope erroneously believed they belonged to the same individual. Based on the fossils found at Garden Park, Cope erected several more genera and species of dinosaur including; Camarasaurus leptodirus (1877), Caulodon diversidens (1877), Caulodon leptoganus (1878), and Amphicoelias latus (1877). All of these species are considered synonyms of C. supremus and were described on the basis of fragmentary or isolated fossils. Many other taxa were named by Cope based on fossils from Garden Park, including Epanterias, Amphicoelias altus, and Hypsirhophus.

More fossils would be found throughout the late 1870s into 1880 by O. W. Lucas and his brother, Ira H. Lucas. All fossils found by I. H. Lucas were then separated into AMNH 5761, however this specimen too included several, jumbled skeletons from multiple individuals. The total number of specimens from Garden Park referred to C. supremus includes: one braincase, one maxilla, one quadrate, two dentaries, several teeth, 23 cervicals, 38 dorsals, two sacra, 102 caudals, 25 chevrons, two sternal plates, 110 ribs, six scapulae, two coracoids, two humeri, two metacarpals, five ilia, eight ischia, six pubes, four femora, four tibiae, two fibulae, one astragalus, and one metatarsal. However, the referral of some of these fossils is questionable, as many are fragmentary or may belong to other sauropods. Intermingled are the remains of possibly six or more individuals, some of which were found alongside bones of theropods and ornithischians. Later, these fossils were prepared and cleaned by Jacob Geismar under Cope's direction between 1877 and the 1890s. Many additional fossils were illustrated and described by Cope in 1878, but a full review of all the Garden Park dinosaur fossils was published by Henry Osborn and Charles C. Mook in 1921.

In 1877, Cope commissioned Dr. John A. Ryder to create a restoration of Camarasaurus using the known fossils of several individuals. Due to using multiple individuals, factors like the vertebral count and proportions were inaccurate. Additionally, Camarasaurus was known only from skull fragments at the time, leading to a largely hypothetical skull which bore carnivorous teeth. This reconstruction would be the first ever made of a sauropod dinosaur and was natural size, measuring over 50 feet (15 m).

Between 1877 and 1879, paleontologists William Reed, Marshall P. Felch, and Samuel W. Williston found an incomplete, partially articulated skeleton (YPM 1901) of a juvenile sauropod in Quarry 1 at Como Bluff, Wyoming. Marsh was sent part of the specimen which he described in 1877 as belonging to a new species of the genus Apatosaurus, naming it Apatosaurus grandis, the specific name coming from the Latin grandis meaning "great". This skeleton would be the best preserved single individual of Camarasaurus known at the time, consisting of: a basioccipital, several dorsals, a partial sacrum, the first 27 caudals, a complete left and incomplete right pectoral girdle, an incomplete left forelimb, femora, tibiae, fibulae, and ribs. Additionally, several paratypes were unearthed from the same locality, including: an incomplete skull and partial postcranial skeleton (YPM 1905), the holotype sacrum of the later named Morosaurus impar (YPM 1900), and a fragmentary postcranial skeleton (YPM 1903).