Stegastes diencaeus, the longfin damselfish, is a damselfish in the family Pomacentridae from the Western Atlantic. It occasionally makes its way into the aquarium trade. It grows to a size of 12.5 cm in length. Longfin damselfish have been reported to have a mutualistic relationship with mysid shrimp of the species Mysidium integrum. The interaction is described as a form of domestication with the shrimp providing nutrients for the algae farms the fish feed on and the fish providing protection from predators.