On the basis of combined analyses of karyotypic, molecular and morphologic data, we herein describe a new Calomys species collected in a transitional area between the Atlantic Forest and the Cerrado morphoclimatic domains of eastern Brazil. This new taxon differs from all other Brazilian Calomys species by its diploid number (2n=38), the lowest among Brazilian Calomys species, and by its yellowish snout. Phylogenetic analyses based on cytochrome b DNA suggest that this species belongs to the larger-bodied species group within Calomys, together with C. expulsus, C. callidus, C. callosus, and C. tocantinsi.