Tuna are a group of salt water fish from the family Scombridae, particularly of the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of 70 km/h (43 mph). Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an oxygen-binding molecule, which tuna express in quantities far higher than most other fish. Some larger tuna species, such as bluefin tuna, display some warm-blooded adaptations, and can raise their body temperatures above water temperatures by means of muscular activity. This enables them to survive in cooler waters and to inhabit a wider range of ocean environments than other types of fish.

Altogether, over fifty different species, all belonging to the family Scombridae, are commonly referred to as tuna. The term tuna derives from Latin thunnus / Greek θύννος, thynnos.

True tuna belong to the genus Thunnus, or "tuna". There are eight species in the genus. Until recently, it was thought that there were only seven species, and that Atlantic bluefin tuna and the Pacific bluefin tuna were subspecies of the same species. In 1999 Collette established, on molecular and morphological considerations, that these are separate species.