A new species of Scoloplax is described with a remarkable new tooth morphology, possessing dentary teeth with four cusps and premaxillary teeth with two to four cusps, features unique among all Loricarioidea and rare among Siluriformes. Scoloplax baileyi, new species, is further distinguished by eight diagnostic features. The new species is found in the small tributaries of Rio Unini, an important tributary to the Rio Negro, and in the Anavilhanas Archipelago, lower Rio Negro, Amazonas State, Brazil. Based on a review of scoloplacids, a new record of S. baskini is given to include the Rio Negro, increasing the number of species of Scoloplax from that basin to four.