Body elongated and depressed. Dorsal body profile gently convex, elevating from snout tip to dorsal-fin origin and descending to first caudal-fin procurrent spine. Greatest body depth at dorsal-fin origin. 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; in some specimens also some isolated platelets near pectoral-fin base.

Head wide and moderately depressed. Head and snout weakly obtuse in dorsal view. A ridge from naris to superior margin of orbit. Snout convex in lateral profile. Eye moderately small (12.2–13.0 of HL), dorsolaterally placed. Lips well developed and rounded. Lower lip almost reaching pectoral girdle and covered by papillae, wider anteriorly; two or three irregular and conspicuous rows of large and transversally flattened papillae, just posterior to dentary teeth. Maxillary barbel short and mostly coalesced with lower lip, generally with free tip. Teeth long, slender and bicuspid; mesial cusp longer than lateral. Dentary rami forming an angle of approximately 120°. Dorsal-fin spinelet half-moon shaped and wider than dorsal-fin spine base. Dorsal-fin with spine flexible, followed by seven branched rays. Moderate to well developed and always present adipose fin. Pectoral fin with six branched rays and a depressed and inward curved spine (more pronounced in larger specimens) shorter than longest branched ray. Pelvic fin with one spine and five branched rays. Pelvic-fin spine ventrally flattened, with dermal flap on its dorsal surface in males. Anal fin with one flexible spine and five branched rays; its posterior margin straight. Caudal fin bifurcate; lower lobe longer than upper; 14 branched rays. Pectoral and pelvic-fin spines with odontodes on lateral and ventral portions. Anal-fin spine with odontodes only ventrally.

Neoplecostomus corumba can be distinguished from N. selenae by lacking enlarged odontodes and distinct swollen skin along lateral margins of snout and along ridges before the eyes in mature males (vs present); from N. yapo by lacking enlarged odontodes and distinct swollen skin along lateral margins of snout (vs present), by having smaller interdorsal length/SL (18.4–20.5 vs 20.7–23.0), and greater mandibullary width/HL (16.3–18.1 vs 14.1–15.2); from N. paranensis by having well-developed adipose fin (vs reduced or absent), greater orbital diameter/HL (12.2–13.0 vs 7.9–12.0), and greater mandibullary width/HL (16.3–18.1 vs 8.4–12.4); from N. espiritosantensis by having 10–18 dentary teeth (vs 19–38), greater cleithral width/SL (24.9–27.6 vs 17.0–19.0), and greater orbital diameter/HL (12.2–13.0 vs 6.0–9.0); from N. franciscoensis and N. ribeirensis by having a well-developed dorsal-fin spinelet, wider than dorsal-fin spine base (vs absent or

narrower than dorsal-fin spine base); from N. granosus by having 27–29 lateral-line plates (vs 34–43), and greater orbital diameter/HL (12.2–13.0 vs 9.0–11.0); from N. microps by having greater orbital diameter/HL (12.2–13.0 vs 8.0–11.0); and from N. variipictus by having smaller caudal peduncle depth/SL (6.0–6.5 vs 7.3–7.8), greater orbital diameter/HL (12.2–13.0 vs 9.1–9.9), 15–24 premaxillary teeth (vs 12–14), and 10–18 dentary teeth (vs 7).