Leporinusis one of the most speciose genera of the order Characiformes, with 81 valid species distributed throughout much of Central and South America. The considerable diversity of this genus has generated extensive debate on its classification and internal arrangement. In the present study, we investigated the species diversity of the genusLeporinusin central northern Brazil, and conclude that six valid species—Leporinus maculatus,Leporinus unitaeniatus,Leporinus affinis,Leporinus venerei,Leporinuscf.friderici, andLeporinus piau—are found in the hydrographic basins of the Brazilian states of Maranhão, Piauí, and Tocantins. We analyzed 182 sequences of the Cytochrome Oxidase subunit I gene, of which, 157 were obtained fromLeporinusspecimens collected from the basins of the Itapecuru, Mearim, Turiaçu, Pericumã, Periá, Preguiças, Parnaíba, and Tocantins rivers. The species delimitation analyses, based on the ABGD, ASAP, mPTP, bPTP, and GMYC methods, revealed the presence of four distinct molecular operational taxonomic units (MOTUs), identified asL. maculatus,L. unitaeniatus,L. affinis, andL. piau(from the Parnaíba River). The bPTP method restrictedL. venereito a single MOTU, and confirmed the occurrence of this species in the rivers of Maranhão for the first time. The separation ofL.cf.fridericiinto two clades and the subsequent formation of different operational taxonomic units was consistent with polyphyly in this species, which indicates the existence of cryptic diversity. The arrangement ofL.cf.fridericiandL. piauin two different clades supports the conclusion that theL. piauspecimens from Maranhão were misidentified, based on their morphological traits, reflecting the taxonomic inconsistencies that exist among morphologically similar species. Overall, then, the species delimitation methods employed in the present study indicated the presence of six MOTUs—L. maculatus,L. unitaenitus,L. affinis,L.cf.friderici,L. venerei, andL. piau. In the case of two other MOTUs identified in the present study, one (L. venerei) is a new record for the state of Maranhão, and we believe that the other represents a population ofL. piaufrom the basin of the Parnaíba River.