The costero (Sotalia guianensis) is found in the coastal waters to the north and east of South America. The common name "costero" has been suggested by Caballero and colleagues due to the species' affinity for coastal habitats. The costero is a member of the oceanic dolphin family (Delphinidae). Physically, it resembles the bottlenose dolphin. However, this species is sufficiently different from the bottlenose dolphin that it is given its own genus, Sotalia. It is also known as the Guyana dolphin.

The costero is frequently described as looking similar to the bottlenose dolphin. However, it is typically smaller, at only up to 210 cm length. The dolphin is colored light to bluish grey on its back and sides. The ventral region is light gray. The dorsal fin is typically slightly hooked. The beak is well-defined and of moderate length.

Researchers have recently shown that the costero has a electroreceptive sense, and speculate this may also be the case for other toothed whales.

Although described as species distinct from the tucuxi Sotalia fluviatilis by Pierre-Joseph van Bénéden in 1864, the costero Sotalia guianensis has subsequently been synonymized with Sotalia fluviatilis with the two species being treated as subspecies, or marine and freshwater varieties. The first to reassert differences between these two species was a three-dimensional morphometric study of Monteiro-Filho and colleagues. Subsequently a molecular analysis by Cunha and colleagues unambiguously demonstrated that Sotalia guianensis was genetically differentiated from Sotalia fluviatilis. This finding was reiterated by Caballero and colleagues with a larger number of genes. The existence of two species has been generally accepted by the scientific community; however, the IUCN still treats both as a single species, Sotalia fluviatilis.