Twenty-one nominal species of Hypostomus are recorded in fresh and brackish waters of the Guianas from the Oyapock to the Essequibo Rivers. One of those, Hypostomus plecostomus, was the first described representative of the genus. However this name was often erroneously applied to numerous other species. The identity of H. plecostomus was here investigated based on a dual approach allying classical morphometric and allozyme analyses of several populations representing respectively 15 and five of the Guianese species. Morphometric analyses revealed four morphological groups among the different populations, namely the H. plecostomus, H. watwata, H. gymnorhynchus, and Boeseman’s species groups. The discriminant model used for a posteriori assignment of the Linnean type specimens of H. plecostomus to an extant species showed that the type series was heterogenous and comprised two species: H. plecostomus and H. watwata. A lectotype was designated to stabilize the taxonomy of H. plecostomus and its junior synonyms H. guacari and H. flava. Hypostomus plecostomus was also recognized based on fixed allelic differences with other Hypostomus species analysed. Its expected heterozygosity (0.091) was relatively high, with little differentiation between populations. Both analyses highlighted that H. plecostomus was a widespread species mostly distributed in the lower part of Guianese rivers. A complementary description of the species was provided. Our analysis of the Hypostomus material from the Oyapock to the Essequibo River basins revealed 15 potentially valid species, and new synonymies for H. plecostomus, H. gymnorhynchus and H. corantijni were proposed. The current distribution of the recognized species was specified.