Blackwater streams harbor fish communities composed of species with morphological and physiological adaptations to these environments. This study analyzed the sexual dimorphism and diet of Scleromystax macropterus, a threatened species endemic to blackwater streams in southeastern Brazil. Analyses revealed sexual dimorphism, particularly in the relative proportions of the pectoral and caudal fins, and a diet predominantly composed of insects and sand, the latter indicating substrate foraging, as previously reported for other species of the same family.