Body relatively short and depressed, ventrally flattened to anal-fin origin, flattened to slightly rounded to caudal fin. Greatest body depth at dorsalfin origin. Dorsal body profile gently convex, elevating from snout tip to dorsal-fin origin and descending to first caudal-fin procurrent spine. Dorsal body surface completely covered by dermal plates, excepting for a naked area around dorsal-fin base. Snout tip naked. Ventral head surface naked except by a plate bearing odontodes in front of gill openings. Abdomen with conspicuous, small dermal platelets between insertions of pectoral and pelvic fins, forming a thoracic shield surrounded by naked areas. Head wide and moderately depressed. Head and snout rounded in dorsal view in larger specimens, and slightly obtuse in smaller ones. Snout gently convex in lateral profile. Mature males with moderately hypertrophied odontodes and swollen skin along lateral margins of snout sides, head anterior portion, and along ridge in front of eye. Eye moderately small, dorsolaterally placed. Lips well developed and rounded. Lower lip relatively small, not reaching pectoral girdle and covered by papillae, wider anteriorly; one or two irregular and conspicuous rows of large and transversally flattened papillae, just posterior to dentary teeth. Maxillary barbel short and coalesced with lower lip, generally with free tip. Teeth long, slender and bicuspid; mesial cusp longer than lateral. Dorsal-fin spinelet half-moon shaped and wider than dorsal-fin spine base. Dorsal-fin with spine flexible. Moderate to well developed adipose fin, generally preceded by one or, rarely, two azygous plates. Pectoral fin with a depressed and inward curved spine (more curved in larger specimens), shorter than longest branched ray. Pelvic-fin spine ventrally flattened, with dermal flap on its dorsal surface in males. Caudal in bifurcate; lower lobe longer than upper; 14 branched rays. Pectoral and pelvic-fin spines with odontodes in lateral and ventral portions. Anal-fin spine with odontodes only ventrally.

Neoplecostomus selenae can be distinguished from its congeners, excepting N. yapo, by having enlarged odontodes on distinct swollen skin along snout lateral margins in mature males (vs absent); from N. yapo it differs by having enlarged odontodes along ridges before the eyes in mature males (vs absent) (ref. 75575).