The capybara (Hydrochoerus hydrochaeris) is a giant cavy rodent native to South America. It is the largest living rodent and a member of the genus Hydrochoerus, of which the only other extant member is the lesser capybara (Hydrochoerus isthmius). Its close relatives include guinea pigs and rock cavies, and it is more distantly related to the agouti, the chinchilla, and the coypu. The capybara inhabits savannas and dense forests and lives near bodies of water. It is a highly social species and can be found in groups as large as 100 individuals, but usually lives in groups of 10–20 individuals. The capybara is not a threatened species and it is hunted for its meat and hide and also for grease from its thick fatty skin.

Its common name is derived from Tupi ka'apiûara, a complex agglutination of kaá (leaf) + píi (slender) + ú (eat) + ara (a suffix for agent nouns), meaning "one who eats slender leaves", or "grass-eater".

The scientific name, both hydrochoerus and hydrochaeris, comes from Greek ὕδρω (hydro "water") and χοῖρος (choiros "pig, hog").

The capybara and the lesser capybara belong to the subfamily Hydrochoerinae along with the rock cavies. The living capybaras and their extinct relatives were previously classified in their own family Hydrochoeridae. Since 2002, molecular phylogenetic studies have recognized a close relationship between Hydrochoerus and Kerodon, the rock cavies, supporting placement of both genera in a subfamily of Caviidae.

Paleontological classifications previously used Hydrochoeridae for all capybaras, while using Hydrochoerinae for the living genus and its closest fossil relatives, such as Neochoerus, but more recently have adopted the classification of Hydrochoerinae within Caviidae. The taxonomy of fossil hydrochoerines is also in a state of flux. In recent years, the diversity of fossil hydrochoerines has been substantially reduced. This is largely due to the recognition that capybara molar teeth show strong variation in shape over the life of an individual. In one instance, material once referred to four genera and seven species on the basis of differences in molar shape is now thought to represent differently aged individuals of a single species, Cardiatherium paranense. Among fossil species, the name "capybara" can refer to the many species of Hydrochoerinae that are more closely related to the modern Hydrochoerus than to the "cardiomyine" rodents like Cardiomys. The fossil genera Cardiatherium, Phugatherium, Hydrochoeropsis, and Neochoerus are all capybaras under that concept.

The capybara has a heavy, barrel-shaped body and short head, with reddish-brown fur on the upper part of its body that turns yellowish-brown underneath. Its sweat glands can be found in the surface of the hairy portions of its skin, an unusual trait among rodents. The animal lacks down hair, and its guard hair differs little from over hair.

Adult capybaras grow to 106 to 134 cm (3.48 to 4.40 ft) in length, stand 50 to 62 cm (20 to 24 in) tall at the withers, and typically weigh 35 to 66 kg (77 to 146 lb), with an average in the Venezuelan llanos of 48.9 kg (108 lb). Females are slightly heavier than males. The top recorded weights are 91 kg (201 lb) for a wild female from Brazil and 73.5 kg (162 lb) for a wild male from Uruguay. Also, an 81 kg individual was reported in São Paulo in 2001 or 2002. The dental formula is 1.0.1.3 1.0.1.3. Capybaras have slightly webbed feet and vestigial tails. Their hind legs are slightly longer than their forelegs; they have three toes on their rear feet and four toes on their front feet. Their muzzles are blunt, with nostrils, and the eyes and ears are near the top of their heads. Its karyotype has 2n = 66 and FN = 102.

Capybaras are semiaquatic mammals found throughout almost all countries of South America except Chile. They live in densely forested areas near bodies of water, such as lakes, rivers, swamps, ponds, and marshes, as well as flooded savannah and along rivers in the tropical rainforest. They are superb swimmers and can hold their breath underwater for up to five minutes at a time. Capybara have flourished in cattle ranches. They roam in home ranges averaging 10 hectares (25 acres) in high-density populations.

Many escapees from captivity can also be found in similar watery habitats around the world. Sightings are fairly common in Florida, although a breeding population has not yet been confirmed. These escaped populations occur in areas where prehistoric capybaras inhabited; late Pleistocene capybaras inhabited Florida and Hydrochoerus gaylordi in Grenada. In 2011, one specimen was spotted on the Central Coast of California.

Capybaras are herbivores, grazing mainly on grasses and aquatic plants, as well as fruit and tree bark. They are very selective feeders and feed on the leaves of one species and disregard other species surrounding it. They eat a greater variety of plants during the dry season, as fewer plants are available. While they eat grass during the wet season, they have to switch to more abundant reeds during the dry season. Plants that capybaras eat during the summer lose their nutritional value in the winter, so they are not consumed at that time. The capybara's jaw hinge is not perpendicular, so they chew food by grinding back-and-forth rather than side-to-side. Capybaras are autocoprophagous, meaning they eat their own feces as a source of bacterial gut flora, to help digest the cellulose in the grass that forms their normal diet, and to extract the maximum protein and vitamins from their food. They may also regurgitate food to masticate again, similar to cud-chewing by cattle. As is the case with other rodents, the front teeth of capybaras grow continually to compensate for the constant wear from eating grasses; their cheek teeth also grow continuously.

Like its relative the guinea pig, the capybara does not have the capacity to synthesize vitamin C, and capybaras not supplemented with vitamin C in captivity have been reported to develop gum disease as a sign of scurvy.

They can have a lifespan of 8–10 years, but tend to live less than four years in the wild due to predation from jaguars, pumas, ocelots, eagles, and caimans. The capybara is also the preferred prey of the green anaconda.

Capybaras are known to be gregarious. While they sometimes live solitarily, they are more commonly found in groups of around 10–20 individuals, with two to four adult males, four to seven adult females, and the remainder juveniles. Capybara groups can consist of as many as 50 or 100 individuals during the dry season when the animals gather around available water sources. Males establish social bonds, dominance, or general group consensus. They can make dog-like barks when threatened or when females are herding young.