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During an expedition to the headwaters of the Rio das Mortes, Mato Grosso, in the Brazilian Shield, a new species of Serrapinnus was found. The new species is distinguished from all congeners by having eight branched dorsal-fin rays, three premaxillary teeth, and a conspicuous dark stripe extending from the eye to the caudal-fin median rays. Among congeners, a similar coloration pattern is present only in S. sterbai, but in that species the stripe usually originates posterior to the eye, on the opercle, and extends to the caudal-fin median rays. In the single locality where the new species was captured, it occurs syntopically with Hyphessobrycon cf. vilmae, another small characid in which a dark midlateral stripe is quite remarkable. Both species were captured together and were observed in situ swimming in mixed shoals. Thus, it is suggested that both species increase their individual chances of survival through the strategy of “protective association” via numeric mimicry.