Body elongated and depressed. Body profile gently convex, elevating from snout tip to dorsal-fin origin and descending to first caudal-fin procurrent spine. Trunk and caudal peduncle dorsally rounded in cross-section; body ventrally flattened to anal-fin origin, flattened to slightly rounded to caudal fin. 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 depressed. Median ridge slightly rising from snout tip to area between nares or not evident. A ridge from naris to superior margin of orbit. Snout gently convex in lateral profile. Mature male with moderately enlarged odontodes and distinct swollen skin along lateral margins of snout. Eye moderately small, dorsolaterally placed. Lips well developed and rounded. Lower lip almost reaching pectoral girdle and covered with papillae. Maxillary barbel short and coalesced with lower lip, generally with free tip. Dorsal-fin origin slightly posterior to vertical passing through pelvic-fin origin; nuchal plate not covered by skin; dorsal-fin spinelet half-moon shaped and wider than dorsal-fin spine base. Dorsal-fin with spine flexible. Adipose fin, preceded by azygous plate. Pectoral fin with inward curved spine (more pronounced in larger specimens). Pelvic-fin spine ventrally flattened, with dermal flap on its dorsal surface in males. Caudal fin bifurcate; lower lobe longer than upper. Pectoral and pelvic-fin spines with odontodes on lateral and ventral portions. Anal-fin spine with odontodes only ventrally.

Neoplecostomus yapo can be diagnosed from its congeners, excepting N. selenae, by having enlarged odontodes and distinct swollen skin along lateral margins of snout in mature males (vs absent); from N. selenae by lacking enlarged odontodes and distinct swollen skin along ridges before eyes (vs present). More distinctive caracteristics in diagnosis in Ref. 55757