Stenolicmus sarmientoi is described as a new genus and species of the trichomycterid subfamily Sarcoglanidinae, from the RíAo Mamoré Basin, Bolivia. It can be distinguished from all other sarcoglanidines by: five-rayed pectoral fin; elongate body shape (HL about 15% of SL); absence of fontanelles on cranial roof; well-developed patches of opercular and interopercular odontodes (five or six on each bone); numerous accessory caudal-fin rays (13 dorsal and 11 ventral); and presence of extensive dark pigmentation on the surface of the body. Three synapomorphies support a hypothesis that the new taxon forms a monophyletic group with other sarcoglanidines: presence of an anterior ossification of the palatine cartilage; quadrate with a dorsal expansion directed posteriorly; and premaxilla with a long lateral process. Stenolicmus lacks some synapomorphies shared by Stauroglanis, Malacoglanis and Sarcoglanis, and shares some other synapomorphies exclusively with Sarcoglanis and Malacoglanis. These character distributions make it uncertain whether Stenolicmus is the sister group of all other sarcoglanidines or only of Malacoglanis plus Sarcoglanis, because the two positions imply equally parsimonious cladograms.