In this contribution, Colacus rubrofemoratus Sobral, Morais, Grossi new species (Coleoptera: Scarabaeidae: Dynastinae: Agaocephalini) is described and illustrated based on male specimens found in a transitional area of Caatinga, Atlantic Forest, and Cerrado in Brazil. In addition, the female of Colacus morio Ohaus, 1910 is redescribed, a distribution map for the genus is provided, and a new key to the species of Colacus is presented.
Keywords: Coleoptera, Agaocephalini, rhinoceros beetles, Scarabaeoidea, taxonomy.