Sauropoda (/sɔːˈrɒpədə/), whose members are called sauropods (/ˈsɔːrəpɒdz/; from sauro- + -pod; lit. 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their body), and four thick, pillar-like legs. They are notable for the enormous sizes attained by some species, and the group includes the largest animals to have ever lived on land. Well-known genera include Alamosaurus, Apatosaurus, Argentinosaurus, Brachiosaurus, Brontosaurus, Camarasaurus, Diplodocus, Dreadnoughtus, and Mamenchisaurus.

The oldest known unequivocal sauropod dinosaurs are known from the Early Jurassic. Isanosaurus and Antetonitrus were originally described as Triassic sauropods, but their age, and in the case of Antetonitrus also its sauropod status, were subsequently questioned. Sauropod-like sauropodomorph tracks from the Fleming Fjord Formation (Greenland) might, however, indicate the occurrence of the group in the Late Triassic. By the Late Jurassic (150 million years ago), sauropods had become widespread (especially the diplodocids and brachiosaurids). By the Late Cretaceous, one group of sauropods, the titanosaurs, had replaced all others and had a near-global distribution. However, as with all other non-avian dinosaurs alive at the time, the titanosaurs died out in the Cretaceous–Paleogene extinction event. Fossilised remains of sauropods have been found on every continent, including Antarctica.

The name Sauropoda was coined by Othniel Charles Marsh in 1878, and is derived from Ancient Greek, meaning "lizard foot". Sauropods are one of the most recognizable groups of dinosaurs, and have become a fixture in popular culture due to their enormousness.

Complete sauropod fossil finds are extremely rare. Many species, especially the largest, are known only from isolated and disarticulated bones. Many near-complete specimens lack heads, tail tips and limbs.

Sauropods were herbivorous (plant-eating), usually quite long-necked quadrupeds (four-legged), often with spatulate (spatula-shaped: broad at the tip, narrow at the neck) teeth. They had relatively tiny heads, massive bodies, and most had long tails. Their hind legs were thick, straight, and powerful, ending in club-like feet with five toes, though only the inner three (or in some cases four) bore claws. Their forelimbs were rather more slender and typically ended in pillar-like hands built for supporting weight; often only the thumb bore a claw. Many illustrations of sauropods in the flesh miss these facts, inaccurately depicting sauropods with hooves capping the claw-less digits of the feet, or more than three claws or hooves on the hands. The proximal caudal vertebrae are extremely diagnostic for sauropods.

The sauropods' most defining characteristic was their size. Even the dwarf sauropods (perhaps 5 to 6 metres (16 to 20 ft) long) were counted among the largest animals in their ecosystem. Their only real competitors in terms of size are the rorquals, such as the blue whale. But, unlike whales, sauropods were primarily terrestrial animals.

Their body structure did not vary as much as other dinosaurs, perhaps due to size constraints, but they displayed ample variety. Some, like the diplodocids, possessed tremendously long tails, which they may have been able to crack like a whip as a signal or to deter or injure predators, or to make sonic booms. Supersaurus, at 33 to 34 metres (108 to 112 ft) long, was the longest sauropod known from reasonably complete remains, but others, like the old record holder, Diplodocus, were also extremely long. The holotype (and now lost) vertebra of Amphicoelias fragillimus (now Maraapunisaurus) may have come from an animal 58 metres (190 ft) long; its vertebral column would have been substantially longer than that of the blue whale. However, research published in 2015 speculated that the size estimates of A. fragillimus may have been highly exaggerated. The longest dinosaur known from reasonable fossils material is probably Argentinosaurus huinculensis with length estimates of 35 to 36 metres (115 to 118 ft) according to the most recent researches. However the giant Barosaurus specimen BYU 9024 might have been even larger reaching lengths of 45–48 meters (148–157 ft). Others, like the brachiosaurids, were extremely tall, with high shoulders and extremely long necks. The tallest sauropod was the giant Barosaurus specimen at 22 m (72 ft) tall. By comparison, the giraffe, the tallest of all living land animals, is only 4.8 to 5.6 metres (16 to 18 ft) tall.

The best evidence indicates that the most massive were Argentinosaurus (65 to 80 metric tons (72 to 88 short tons)), Mamenchisaurus sinocanadorum (60 to 80 metric tons or 66 to 88 short tons), the giant Barosaurus specimen (60-80+ metric tons) and Patagotitan with Puertasaurus (50 to 55 metric tons (49 to 54 long tons; 55 to 61 short tons)). Meanwhile, 'mega-sauropods' such as Bruhathkayosaurus has long been scrutinized due to controversial debates on its validity, but recent photos re-surfacing in 2022 have legitimized it, allowing for more updated estimates that range between 110 to 170 metric tons (120 to 190 short tons), rivaling the blue whale in size. The weight of Amphicoelias fragillimus was estimated at 122.4 metric tons (120.5 long tons; 134.9 short tons) tons with lengths of up to nearly 60 meters (200 ft) but 2015 research argued that these estimates were based on a diplodocid rather than the more modern rebbachisaurid, suggesting a much shorter length of 35–40 meters (115–130 ft) with mass between 80 and 120 metric tons (79 and 118 long tons; 88 and 132 short tons). Additional finds indicate a number of species likely reached or exceeded weights of 40 tons. The largest land animal alive today, the bush elephant, weighs no more than 10.4 metric tons (11.5 short tons).