The taxa of the genus Megasoma Kirby, 1825 ( Coleoptera , Scarabaeidae , Dynastinae ) related to M. gyas (Jablonsky in Herbst, 1785) are revised. Megasoma ( M.) gyas is recognized as a monotypic species restricted to the Caatinga biome of northeastern Brazil. Megasoma gyas rumbucheri Fischer, 1968, is considered as a new synonym of M. gyas . The “long-horned M. gyas ” is recognized as a separate polytypic species M. ( M.) typhon (Olivier, 1789) with the nominative subspecies occurring through the Mata Atlântica biome of Brazil, from Bahia to São Paulo states and M. ( M.) typhon prandii Milani, 2008 restricted to a small area in the state of Santa Catarina, South Brazil. Megasoma gyas porioni Nagai is considered as a new synonym of M. typhon typhon . The “short-horned M. gyas ” occurring in Minas Gerais, São Paulo, and southwestern Bahia, is recognized as a separate new species and described as M. ( M.) hyperion sp. nov. The paper includes an historical research and the redescriptions of the other nominal species of the genus. Distribution maps and a key to species in the M. ( M.) gyas species group (males and females) are also provided.