The brown trout is a species of salmonid ray-finned fish and the most widely distributed species of the genus Salmo, endemic to most of Europe, West Asia and parts of North Africa, and has been widely introduced globally as a game fish, even becoming one of the world's worst invasive species outside of its native range.

Salmo is a genus of ray-finned fish from the subfamily Salmoninae of family Salmonidae, and is part of the tribe Salmonini along with the sister genera Salvelinus and Salvethymus. Almost all Salmo species are native only in the Old World, the only exception being the Atlantic salmon, which is also naturally found across the North Atlantic in eastern North America.