Cnesterodon pirai is diagnosed by the combination of the following characters: 6 to 8 irregular dashes on flanks, ranging from oval to vertical stripes on females and 7 to 9 irregular dashes ranging from oval to circular dots on males (dashes both on males and females covering 1 or 2 scales in transverse row); absence of a large post-gonopodium blotch on ventral profile in adult males; lack of a distal filament on

gonopodium; absence of longitudinal dark-brown band along flank; snout pointed and long (16.7–28.7 %HL); possession of 12–13 epipleural ribs; medial surface of ascending process of premaxilla approximately straight; presence of teeth on fourth ceratobranchial; distal portion of third and fourth gonactinosts separate, except by tip of third gonactinost; fifth gonactinost free; and presence of a constriction on unpaired appendix of gonopodium (Ref. 82179).