A new species of Moenkhausia is described from the upper Rio Juruena, Rio Tapajós basin, Mato Grosso, Brazil. The new species is diagnosed from all congeners, except M. eurystaenia, M. heikoi, and M. phaeonota, by the presence of a dark, straight, broad stripe extending from the opercle to the middle caudal-fin rays. The new species can be distinguished from M. heikoi and M. phaeonota by having 18–20 branched anal-fin rays and from M. eurystaenia by having a narrow longitudinal dark line along the horizontal septum, starting at the vertical through the pelvic-fin origin. The presence of a conspicuous dark blotch on the ventral portion of the opercle and on the ventral portion of the vertical arm of the preopercle is also useful to distinguish the new species from most congeners.