Abstract
A new species of the small‐sized armoured catfish genus Neoplecostomus is described from Neotropical Atlantic coastal drainages, in southeast Brazil. The new species can be readily diagnosed from all congeners by having the first plates of the ventral series reduced in size, rounded and ventrally displaced, resulting in a naked band between the mid‐ventral and ventral series of plates. Additionally, the species exhibits marked sexual dimorphism in dentition: Females possess numerous slender teeth, while males have fewer, more robust teeth. We report the second known case of sympatry between congeneric species of Neoplecostomus , a rare condition for the genus, and provide insights into its biogeographic and ecological patterns.