The genus Isometocoris Carvalho and Sailer is reviewed and I. penicillus, new species, from Brazil is described. Diagnoses of the genus and included species I. blantoni Carvalho and Sailer and I. penicillus, n. sp., are given; a color adult habitus photo of both Isometocoris species, male genitalic drawings of Isometocoris penicillus n. sp., and scanning electron micrographs of selected structures of I. blantoni are provided; and an identification key is given to help distinguish the two included species. Isometocoris is transferred from the subfamily Psallopinae to the Cylapinae, where it is placed in the tribe Fulviini. Psallopinae is reduced to tribal level (Psallopini, new status) within the Cylapinae, and relationships among Cylapinae, Isometopinae, and Psallopinae are discussed.