The genus Reigechimys (Rodentia, Echimyidae) was originally described on the basis of two mandibular fragments recovered from Huayquerian (late Miocene) outcrops of the Cerro Azul Formation in La Pampa Province (Argentina). The included species (R. plesiodon Verzi et al. and R. octodontiformis Verzi et al.) represent a peculiar lineage characterized by its molar morphology with occlusal simplification that is extreme for the family. New materials recently found at the Cerro Azul Formation allow for a more detailed knowledge of the variation of Reigechimys and recognition of a new species, Reigechimys simplex sp. nov. characterized by the absence of mesoflexid/fossettid in the m1–2. The morphological changes of Reigechimys are described in the context of the turnover exhibited by octodontoid rodents in the Huayquerian of the Cerro Azul Formation. The polarity of changes described for Reigechimys is consistent with those of the other lineages. This evolutionary pattern enhances the biochronological interpretation of the Huayquerian in this formation.