Astyanax xiru can be diagnosed from all species of Astyanax from the rio Uruguay basin and laguna dos Patos and rio Tramandaí systems by having the following characters: two humeral spots; shape of the first humeral spot is on the upper portion horizontally elongate and lower portion vertically narrow (vs. one spot in A. eigenmanniorum, A. sp. aff. fasciatus, A. jacuhiensis, and A. laticeps and ovale shape in last two species); one maxillary tooth (vs. 2-5 in A. henseli and 3-4 in A. paris); maxillary tooth tri- to pentacuspid (vs. heptacuspid in A. ojiara and A. dissensus); anal fin with 18-22 branched rays (vs. 25-30 in A. saguazu and A. stenohalinus, 23-29 in A. aramburui, 18 or less in A. brachypterygium and A. cremnobates); body depth 32.3-37.5% of SL (vs. 38.3-46.0% of SL in A. procerus); very few small hooks on anal and pelvic-fin of males (vs. absence in A. obscurus). It is most similar to A. procerus in color pattern, but it can be distinguished by the pelvic-fin length (13.6-17.1% vs. 16.1-18.7% of SL), dorsal-fin length (21.1-25.4% vs. 24.1-28.7% of SL), males with bony hooks on the anal and pelvic fins (vs. absence). The following combination of characters separate this species from all congeners including the species complex A. bimaculatus and A. scabripinnis: two conspicuous humeral spots (the first humeral spot vertically elongate with upper portion wider and lower portion narrow); a conspicuous wide lateral band; 18- 22 branched anal-fin rays; 19-24 gill rakers on first branchial arch; 37-40 perforated lateral line scales, 4-5 scale row between lateral line and pelvic-fin origin; outer row of premaxilla with pentacuspid teeth, inner premaxilla row with heptacuspid teeth, one maxillary teeth tri- to pentacuspid; very few small hooks on anal and pelvic fin of males, conspicuous caudal spot, head length 24.2-27.3% of SL, body depth 32.5-37.5% of SL, interorbital width 29.8-34.2% of HL, orbital diameter 32.1-36.6% of HL, head length 24.2-27.3% of SL, and anal-fin base length 23.3-27.4% of SL (Ref. 94153)