A new species of the colubrid genus Geophis is described from the Sierra Madre del Sur of Guerrero, Mexico. The new species possesses all of the diagnostic characters of the sieboldi group, but differs from all of the other species in the group by having dorsal scales arranged in 17 rows, 133–139 ventrals in females and 130–137 in males, 24–31 subcaudals in females and 34–39 in males, keeled dorsal scales on at least the posterior three-fourths of the body, apparently no apical pits, and a light nuchal collar in juveniles. The new species fills a gap in the distribution of the sieboldi group between Michoacán and Oaxaca, and is the first Mexican species of the genus reported to exhibit both a unicolor (uniformly dark) and a bicolor (with dark markings on a red background color) body and tail dorsal patterns, a polymorphism similar to that previously documented in few lower Central American species.