Based on a right maxillary with 1st molar recovered in Pleistocene (Ensenadan) deposits from south-central Buenos Aires Province, Argentina, a new genus and species of Oryzomyini (Rodentia, Cricetidae) is named and described. The new taxon is one of the largest known extinct or extant sigmodontines, morphologically related to the marsh rats Holochilus, †Noronhomys, and Pseudoryzomys. It can be differentiated from these taxa by the combination of several traits in the 1st upper molar, in particular a well-developed mesoloph and the free connection of both para- and protoflexus. The occurrence of this sigmodontine suggests warmer and probably moister conditions during deposition times, a hypothesis reinforced by other vertebrates exhumed together (e.g., turtles, coypus, and the giant armadillo †Propraopus).