Serrapinnus potiguar, new species, is described from the rio Ceará-Mirim, a coastal drainage in the Rio Grande do Norte State, northeastern Brazil. The new species is distinguished from the other species of the genus by the shape and arrangement of the ventral procurrent caudal-fin rays of the sexually dimorphic males; where the hypertrophied elements present the shape of a series of scimitars arranged radially, forming a semi-circle on the ventral margin of the caudal peduncle. Furthermore, the new species is diagnosed from S. heterodonand S. piaba, sympatric congeners from the northeastern Brazilian drainages, respectively by the presence of incomplete lateral line and teeth bearing at most five cusps.