Hypostomus formosae n. sp. is described for the Paraguay river basin, Argentina. It can be distinguished from all other Hypostomus by having the combination of the following features: dorsum of head, body and all fins bright grey covered by numerous rounded dark dots and 11 ventral plates between end of anal-fin base and caudal fin (vs. more than 11). In addition, the new species is distinguished from the most similar sympatric species, H. boulengeri by slightly higher cleithral width, dorsal-fin base length, caudal-peduncle depth and by narrower caudal-peduncle length, eye diameter and mandibulary ramus length. Hypostomus formosae belongs to the Amazonian "H. plecostomus species group" and our molecular phylogenetic results show that it is the sister species to the Guianese H. plecostomus, highlighting past inter-basin ichtyofauna exchanges. We also provide detailed record of H. boulengeri for the first time in Argentina, collected in Paraguay river, at Formosa city.Instituto de Limnología "Dr. Raúl A. Ringuelet
