Astyanax puka is distinguished from the remaining species of the genus by the following combination of characters: shallow body (33.5–39.8% SL), 36–38 perforated scales in lateral line, 6/5 transverse scales, 9–10 gradually decreasing dentary teeth, distally expanded premaxillary teeth, one distally expanded maxillary tooth with 7–9 cusps, short upper-jaw length (34.5–37.8% HL), short maxilla (20.7–23.3% HL)expanded abruptly under maxillary tooth position, short postorbital distance (36.6–46.9% HL), short pelvic to anal-fin origin distance (17.1–20.6% SL), 22–27 branched anal-fin rays, long caudal peduncle (13.3–16.5% SL), bony hooks restricted to pelvic-fin rays 2 to 6 and anterior 6–13 branched anal-fin rays on mature males, and the presence of broad denticles on anterior, lateral, and posterior edges of first ceratobranchial gill rakers (Ref. 72407).