The tucuxi (Sotalia fluviatilis), alternately bufeo gris or bufeo negro (both in Peru) is a dolphin found in the rivers of the Amazon Basin. The word tucuxi is derived from the Tupi language word tuchuchi-ana, and has now been adopted as the species' common name. Despite being found in geographic locations similar to those of 'true' river dolphins, such as the boto, the tucuxi is not closely related to them genetically. Instead, it is classed in the oceanic dolphin family (Delphinidae). Physically, the species, particularly the marine variety, resembles the bottlenose dolphin. However, this species is sufficiently different from the bottlenose dolphin that it is given its own genus, Sotalia. Animals formerly called the tucuxi occurring in coastal and estuarine environments have recently been recognized as a distinct species, the costero (Sotalia guianensis), also known as the pink dolphin.

The tucuxi is frequently described (see references below) as looking similar to the bottlenose dolphin, but it is typically smaller at around 150 cm. The dolphin is coloured light to bluish grey on its back and sides. The ventral region is much lighter, often pinkish. The dorsal fluke is typically slightly hooked. The beak is well-defined and of moderate length. There are 26 to 36 pairs of teeth in the upper and lower jaws.