The giant otter is a member of the weasel family, Mustelidae, in the Mammalian Order of Carnivora. Other mustelids include ferrets, mink, skunks, badgers, and wolverines. The river otter's scientific name is Pteronura brasiliensis, which means "winged tail belonging to Brazil", although this otter species is not limited to Brazil. Winged refers to the fin-like shape of the giant otter's tail.

Despite its name, this species is not the largest otter species. The giant otter shares that title with the sea otter. The giant otter is the longest otter in the world, while the sea otter is the heaviest. Adult otters can range from 5-6ft in length and weigh from 50-70lbs.

Giant otters are found in freshwater rivers, creeks and lakes in the South American rainforests of Brazil. Their preferred prey consists of slow-swimming fish, such as perch and catfish, crustaceans and small snakes. Giant otters have also been known to eat small caiman—a relative of alligators—when they cannot find enough fish. The adults may consume 6 to 9 pounds of food each day.

Completely diurnal, these large animals live and hunt in family groups called holts. A holt may consist of as many as 10 individuals, led by an adult breeding pair. The members will hunt together in deeper water, earning the giant otter the local nickname of “River Wolf.” Kits are born in early fall. Gestation is 65-70 days. One to five young are born, weighing four ounces each. Solid food is eaten at 3-4 months of age. The young are weaned sometime after the next year's litter are born, but usually remain with the family group for a period time after.

As a top predator, the giant otter does not have many natural threats. However, their numbers have declined due to human impacts, and these animals are now considered to be a vulnerable species. The most direct threats from humans arise from habitat destruction, over fishing, and chemical pollution of the water. Giant otters are also susceptible to parvovirosis, a disease carried by dogs and cats. The otters used to be hunted as well, but today there are laws to protect them from poachers.