The genus Oligoryzomys is the most speciose genus in the tribe Oryzomyini; however, under the general lineage concept of species, at least nine lineages are pending review and naming. In this study, I carry out qualitative and quantitative morphological analyses of specimens of Oligoryzomys andinus, O. aff. andinus, and O. arenalis, to delimit these three lineages. I show that O. aff. andinus is morphologically distinct both from O. andinus and O. arenalis, corroborating that the lineage merits recognition as a species, as suggested by previously published molecular analyses (Hurtado and D'Elía 2019). I therefore describe O. aff. andinus as a new species endemic to the Polylepis forests on the western slope of the Andes and coastal Lomas in central Peru. Finally, I propose two hypotheses about the biogeographical history of O. andinus, O. arenalis, and this new species.