Neoplecostomus langeanii differs from N. bandeirante by the absence of keels along each lateral series of plates (vs. presence of moderate keels). Neoplecostomus langeanii differs from N. botucatu and N. paranensis, by having a well-developed adipose fin (vs. adipose fin reduced or absent). Neoplecostomus langeanii differs from N. franciscoensis and N. ribeirensis, by having a dorsal-fin spinelet larger than dorsal-fin spine (vs. dorsal-fin spinelet absent to vestigial and more slender than dorsal-fin spine). Neoplecostomus langeanii differs from N. granosus, N. microps, and N. variipictus by the presence of five conspicuous dark bands on dorsum and lacking evident dark spots (vs. conspicuous dark spots all over body and fins and dorsal bands not evident) characteristics that are more evident in mature adults. Neoplecostomus langeanii is distinguished from N. selenae by lacking enlarged odontodes and a distinct, swollen integument along lateral margins of snout and along ridges anterior to eyes (vs. having enlarged odontodes and distinct swollen integument along lateral margins of snout and along ridges anterior to eyes). Neoplecostomus langeanii is distinguished from N. corumba by having smaller orbital diameter 8.3-11.4% in head length, 12.9–18.5% in snout length, and 27.0–35.7% in interorbital length (vs. 12.2–13.05%, 18.4–20.1%, and 36.7–41.5%, respectively); from N. yapo, by having a smaller interdorsal length, ranging from 14.8–19.5% in SL (vs. 20.7–23.0%), greater caudal peduncle depth 19.8–29.0% in caudal peduncle length (vs. 17.6–19.6%), smaller orbital diameter 8.3–11.4% in head length (vs. 11.9–21.4%); from N. espiritosantensis, by having greater cleithral width 25.6–30.0% in SL (vs. 19.0–21.0%) (Ref. 89726).