The alligator gar (Atractosteus spatula) is a euryhaline ray-finned fish in the clade Ginglymodi of the infraclass Holostei /hoʊˈlɒstiaɪ/, being most closely related to the bowfins. It is the largest species in the gar family (Lepisosteidae), and is among the largest freshwater fishes in North America. The fossil record traces its group’s existence back to the Early Cretaceous over 100 million years ago. Gars are often referred to as “primitive fishes” or “living fossils”, because they have retained some morphological characteristics of their early ancestors, such as a spiral valve intestine, which is also common to the digestive system of sharks, and the ability to breathe in both air and water. Their common name was derived from their resemblance to the American alligator, particularly their broad snouts and long, sharp teeth. It is suggested that an alligator gar can grow up to 10 ft (3.0 m) in length.

The body of an alligator gar is torpedo-shaped, usually brown or olive colored, fading to a lighter gray or yellow ventral surface. In very rare occurrences, they can also be black, seen in gars that have a high level of melanin.