Astyanax taeniatus is distinguished from all other species of the Astyanax by the presence of a gap between the symphyseal dentary teeth, with the exception of A. henseli. It differs from A. henseli by having only one humeral spot (vs. two), 19-23 anal-fin rays (vs. 22-27), orbital diameter 38.5-46, mean ¼ 42.2 (vs. 42.4-50.7, mean ¼ 46.7), and 12-13 gill rakers on hypobranchial (vs. 14-15). In addition, Astyanax taeniatus differs from A. henseli and most Astyanax species, except A. ribeirae, A. hastatus, A. giton, and A. bahiensis, by having a humeral spot shaped like a comma. It can be diagnosed from A. ribeirae and A. hastatus by having 38-41 scales on lateral line (vs. 34-36) and from A. giton and A. bahiensis by the format of third infraorbital (with a naked area in relation to preopercle vs. reaching preopercle, without naked area). It can be further diagnosed from morphologically similar species of the genus Astyanax that occur in Atlantic coastal river basins by having 38-41 perforated scales in the lateral line (vs. 34-38 in most of species), except A. aff. fasciatus, A. parahybae, and A. pelecus. It differs from A. aff. fasciatus, A. parahybae and A. scabripinnis by the dentary teeth decreasing abruptly after the fifth or sixth tooth (vs. dentary teeth decreasing abruptly after the fourth tooth). It can be differentiated from A. pelecus by humeral spot surpassing lateral line (vs. humeral spot restrict to the region above lateral line) (Ref. 119398).
Teeth in the lower jaw 14 or more. The ventrals are exactly aligned with the origin of the dorsal fin.