Curculionichthys scaius, new species, is distinguished from other congeners by having two usually conspicuous, somewhat triangular dark brown speckles lateral to the anal-fin origin, a pointed snout with paired rostral plates, numerous lateral abdominal plates, a single series of large median abdominal plates, numerous dentary and premaxillary teeth, and several morphometric proportions of the body and head, mostly involving prepelvic, prepectoral, and preanal distances, head and cleithral width, caudal peduncle length, and dorsal-fin spine. The new species represents the first record of the genus Curculionichthys in the Madeira River basin and is the most westerly distribution in the Amazon.