A new species of gopher, Gregorymys mixtecorum n. sp., is described from the Arikareean 1 (early Oligocene) of Oaxaca, southern Mexico. Phylogenetic analysis indicates that it is the sister species of G. veloxikua, which was also recently described from southern Mexico. Both species were collected from sediments of the Chilapa Formation that crop out in northwestern Oaxaca. Gregorymys mixtecorum n. sp. and G. veloxikua show differences in size and proportions that possibly reduced competition for resources, exploiting different microhabitats. Both Mexican species represent the oldest and the most southern records of Gregorymys in North America. The Mexican record of Gregorymys suggests that at least some entoptychine rodents diversified in southern Mexico or Central America, and that Geomyidae has had a wide geographic distribution in North America since the early Oligocene.