Tembeassu marauna is a species of weakly electric knifefish in the family Apteronotidae and the only member of its genus, known only from three specimens collected from the upper Paraná River, Brazil, in 1965. This fish can be identified by fleshy extensions at the tips of its upper and lower jaws, with the upper extension bearing a patch of extra teeth. The function of these unique structures is unknown, but may relate to feeding. Apparently a specialized inhabitant of deep riverine environments, T. marauna may be endangered by extensive dam construction in the upper Paraná region.