We describe an extinct murid, assigned to the tribe Phyllotini, from the late Pleistocene (Tafí del Valle Formation) of La Angostura (26°55′30″S, 65°41′50″W; 1,900 m elevation) in northwestern Tucumán Province, Argentina. The new genus is characterized by very hypsodont molars with flat crowns and simplified occlusal structure, upper incisors with a mediolateral groove, a straight premaxillary–maxillary suture, and high zygomatic plate with a small spine on its anterodorsal edge. Phylogenetic analyses including fossil and living members of the Reithrodon group show that the new genus is the sister taxon to the Reithrodon–Neotomys–Euneomys clade. The paleoenvironmental and paleogeographic significance of the new genus is discussed within the context of the climatic changes that occurred during the late Pleistocene in southern South America. The new phyllotine would have lived in high-elevation grasslands, which today occur >1,000 m higher under cold and dry climatic conditions than those of the last glacial maximum.