Orizabus australis Ratcliffe and Solís is described as a new species from Costa Rica. It resembles Orizabus hondurensis Ratcliffe and Cave from Honduras and is distinguished from that species. Species of Orizabus Fairmaire are known from the central United States to Honduras, but O. australis represents the first known occurrence for the genus in Costa Rica where it represents the southernmost occurrence of the genus. A revised key to the genera of Pentodontini of Costa Rica is provided, and O. australis is incorported into emended couplets of previously published keys to males and females of Orizabus species.