Astyanax turmalinensis differs from the other species of the genus by the following combination of characters: presence of 20 to 24 anal fin rays; 34 to 36 scales in lateral lines; 1 or 2 teeth in the maxillary bone; greatest body depth at pelvic fin origin; caudal peduncle oval spot horizontally elongated, prolonging into median caudal fin rays, onward becoming a dark band laterally on the body, tapering anteriorly, to a point slightly anterior to middle of standard length; humeral black spot present, truncate anteriorly, usually circular posteriorly, extending anteroventrally, followed by a clear area and then subsequentely a vertically elongated dark area of loosely-assembled chromatophores. Snout slender, about the same mength as eye diameter; four teeth in the outer row of the premaxillary bone; posterior border of anal fin concave anteriorly.