A new monotypic genus of Naupactini (Coleoptera: Curculionidae), Thoraconaupactus del Rio and Lanteri is described based on the new species Thoraconaupactus vaninii del Río and Lanteri (Type locality: Brazil, Bahia, Itaji). Two different morphotypes are distinguished by characters of the vestiture: one from Itaji, with integument brown, smooth, and vestiture mostly setose; the other from Ibiraba with integument black, rugose and vestiture mostly squamose. Thoraconaupactus is similar to Naupactus Dejean and Teratopactus Heller in several characters, and shows some unique features such as the strongly transverse pronotum with a sub-basal tooth-like projection on each side. Specimens were collected in association with a species of Leucaena (Fabaceae). The paper includes habitus photographs and line drawings of genitalia, mouthparts, and other external features of taxonomic value.