A new species of Akodon (sigmodontine rodents of the tribe Akodontini) is described from the Córdoba Montane Savanna Ecoregion in the eastern mountain ranges of the Sierras Centrales in central Argentina. Phylogenetic relationships among the new species and 11 other species of Akodon were examined using nucleotide sequence data from the entire 1,140 base pairs of the mitochondrial cytochrome-b gene. Maximum-parsimony, maximum-likelihood, and Bayesian analyses found the new species to be distinct from other Akodon and sister to a clade composed of A. spegazzinii, A. boliviensis, and A. oenos. We present analyses of morphological data and descriptions of external, cranial, and dental morphology. This species is relevant to the insular biogeography of the Sierras Centrales, the uniqueness of the ecoregion, and the level of plant and animal endemism in isolated montane regions of central Argentina.