Moenkhausia bonita, new species, is described from the Rio Baía Bonita, a tributary of the Rio Miranda, Rio Paraguay basin, Bonito, Mato Grosso do Sul, southwestern Brazil. The new species is distinguished from all congeners by the combination of the presence in alcohol-preserved specimens of a black midlateral stripe, a black posterior margin on each caudal fin lobe, the presence of a black lozenge-shaped blotch on the caudal peduncle that extends to the posterior limit of the median caudal fin rays, and the presence of seven gill rakers on the upper limb and 12 gill rakers on the lower limb of the first gill arch. Comments are included on the phylogenetic placement, ecology, and behavior of the species.