A new significant mammal fossil from the Marino Formation in the area of Divisadero Largo (Mendoza, Argentina) is described herein. The material consists of a mandibular fragment with the right series p4-m3 of the rodent Scleromys sp. It is compared with the Santacrucian species S. osbornianus Ameghino and S. angustus Ameghino, and with S. quadrangulatus Kramarz from the Pinturas Formation, all of them from the Santa Cruz Province. At the same time, the studied fossil presents a combination of characters that differ from those species: more transverse molars, with anterior wall slightly curved, lower degree of hypsodonty and smaller size. However, due to the great morphological variation within Scleromys, a specific determination is not provided based on a sole specimen. The presence of this genus, together with the previously studied mesotheriine, supports an early Miocene age for the middle member of the Marino Formation