Hyphessobrycon isiri sp. n., is described from the río Uruguay basin. Hyphessobrycon isiri sp. n. differs from the remaining species of the genus by the presence of teeth of the inner series of the premaxilla with 5 to 11 cusps, one maxillary tooth with 7 cusps; 16 to 22 branched anal-fin rays; one humeral spot vertically elongated, and a midlateral stripe ending in a conspicuous caudal spot.