Diapoma nandi is described from the Piray-Miní stream, a tributary of the Rio Paraná in Argentina. It is characterized among the Stevardiinae by having a terminal mouth, two unbranched and eight branched dorsal-fin rays, one unbranched and six branched pelvic-fin rays and the absence of a caudal-fin organ and is distinguished from all congeners by the following combination of characters: unmodified scales on the lower caudal-fin lobe, lack of enlarged opercle and subopercle, incomplete lateral line, hyaline adipose fin, anal-fin distal border straight or slightly convex in adult males, large tricuspid teeth on anterior region of the dentary, distal arrangements of the anal-fin bony hooks in adult males, middle caudal-fin rays lacking large round blotch and several morphometric variables associated with body shape. Additionally, we conducted a morphometric comparison focused on the congeners that co-occur in the Paraná basin.