Nemadoras cristinae is described from the upper Amazonas/Solimões and Madeira basins in Brazil, Colombia and Peru, with additional specimens from the upper Meta (Orinoco basin), Colombia, tentatively treated as conspecific. Nemadoras cristinae is distinguished from congeners by having 3–9 premaxillary teeth in approximately two rows in juveniles and adults, outermost teeth weakly spatulate and innermost more acicular (vs. premaxillary edentulous in adults of all congeners and limited to 1–6 acicular teeth in juveniles of N. elongatus, N. humeralis, N. leporhinus and N. ternetzi); and mental barbels with extremely elongate (filiform) papillae, length of longest papillae about 4–7 times its width at base (vs. papillae shorter, length of longest <3 times its width at base). Nemadoras cristinae most closely resembles N. leporhinus, but is further distinguished from that species by having shaft and primary fimbriae of maxillary barbel smooth (vs. outer margin of shaft and margins of primary fimbriae with distinct secondary fimbriae). Nuptial specimens of N. cristinae exhibit sexual dimorphism; in males the pungent dorsal spine is greatly prolonged by a soft, flexible tip approximately 19.8–23.9% of the total dorsal spine length (vs. 5.3–9.4% in females). The sturdy, spatulate teeth in the upper and lower jaws of N. cristinae are evidently effective tools for raking caddisfly larvae from their attached substrates, as gut contents were dominated by cases and larval remnants of Nectopsyche (Leptoceridae). The six other species of Nemadoras are redescribed, the monophyly of the genus is discussed, and an identification key is provided. Detailed observations on the mesethmoid and infraorbital bones and associated sensory canal are reported for Nemadoras and compared to other species of fimbriate-barbel doradids.