Cycads are a group of plants in the order Cycadales, comprising ten genera, and with a worldwide distribution. Very few taxonomic revisions are available for the genera in this order, and the present one is intended to provide a revision of the current knowledge in the genus Dioon. The revision was based on bibliographic research and analysis of herbarium specimens, including live specimens found in different botanical gardens and greenhouses. Sixteen species are recognized for the genus. This study includes morphological descriptions, maps with species distribution, line drawings of diagnostic characters, and a key to the species. Detailed taxonomic notes for each species, as well as information on their distribution and habitat is provided, including the uses and common names when available. Fifteen of the total species are distributed in Mexico and one species is found in the border of Honduras and Nicaragua. Therefore, the genus is endemic to Megamexico II.