The giant otter or giant river otter (Pteronura brasiliensis) is a South American carnivorous mammal. It is the longest member of the weasel family, Mustelidae, a globally successful group of predators, reaching up to 1.7 metres (5.6 ft). Atypical of mustelids, the giant otter is a social species, with family groups typically supporting three to eight members.
The groups are centered on a dominant breeding pair and are extremely cohesive and cooperative. Although generally peaceful, the species is territorial, and aggression has been observed between groups.