Briarius [Fischer de Waldheim], senior synonym of Lamprocyphus Marshall, includes some of the most beautiful species of weevils within the Entiminae, Naupactini. It is closely related to Trichaptus Pascoe and according to the present revision has only two species, B. elegans (Roelofs) (new comb.), endemic to northeastern Brazil (Bahía and Espíritu Santo), and B. augustus (Illiger) distributed in northeastern Argentina, Brazil south of the Amazon River, and eastern Paraguay, in tropical and subtropical forests. After the examination of about 700 specimens from several localities we conclude that Briarius augustus is a complex species with population polymorphism (mainly in color vestiture) and geographic variation. Within it we have distinguished four subspecies previously described as species: B. a. augustus (Illiger) (= Cyphus oliveirae Roelofs, new syn.) occurs in Santa Catarina and Paraná on the Serra do Mar (Brazil); B. a. germari (Boheman) ranges from São Paulo to Espirítu Santo (Brazil), along the Atlantic forest; B. a. margaritaceus (Sturm) (= Cyphus consularis Chevrolat, C.. gloriandus Schoenherr, and C. spixi Perty) is distributed from Santa Catarina to Bahía (Brazil); and B. a. varnhageni (Germar) (= Lamprocyphus jacobi Hustache) ranges from Santa Catarina to northeastern Argentina (Misiones) and eastern Paraguay. Subspecies have overlapping ranges on the Serra do Mar (Santa Catarina), where it is possible to see all the variation within B. augustus. This area, same as other coastal hills along the Atlantic forests of Brazil, would have been Pleistocene refuges where species inhabiting rainforests survived and diversified during the dry-warm cycles of this period, however, time since then was probably not enough to achieve reproductive isolation, since individuals of different subspecies of B. augustus were found in copula. The paper includes descriptions and illustrations of the two valid species of the genus, and the subspecies of B. augustus. It also provides dichotomous keys, habitus photographs, illustrations of genitalia, external morphology, and a map of distribution.