A new Cardiomyinae rodent (Hydrochoeridae) from Arroyo Seco de la Frazada, Mendoza Province, Argentina, is described. The new species, Caviodon cuyano sp. nov., is distinguished by its greater robustness, deeper fissures, and greater number of prisms in M3, which are derived characters within the context of Hydrochoeridae evolution. An assessment of the species of Caviodon Ameghino showed that tooth growth is allometric and occlusal morphology becomes more complex during ontogeny, as in hydrochoerines. The true diversity of this genus, when tooth postnatal ontogeny is considered, may be less than heretofore estimated. The fossil-bearing levels, the middle section of the Aisol Formation, have been considered no older than Huayquerian in age (late Miocene). The evolutionary stage of the new species suggests that the age of these sediments could be even younger, no older than early Pliocene and possibly up to late Pliocene.