The Cerro Bandera Formation is a mostly pyroclastic continental unit exposed in isolated areas of east-central Neuquén Province, northwestern Patagonia. Several mammals from these deposits were previously reported and support a Colhuehuapian age (early Miocene) for these levels. New findings reveal that this unit bears a much more diverse assemblage and calls into question this age assessment. Here we describe three new species. Protypotherium sinclairi sp. nov. (Notoungulata, Interatheriidae) is similar in size to the Santacrucian Protypotherium australe Ameghino, but differs from all the species of the genus by having less reduced p3–p4 talonids. Doryperimys olsacheri gen. et sp. nov. (Rodentia, Neoepiblemidae) has hypselodont, prismatic cheek teeth that combine features of species of Perimys Ameghino and Scotamys antiquus Loomis. The systematic position of Leucokephalos maior sp. nov. within Caviomorpha is still uncertain, but it is definitively allied to the Deseadan Leucokephalos zeffiae Vucetich, Dozo, Arnal, and Pérez in sharing rooted cheek teeth with an almost identical early occlusal simplification that resembles the 8-shaped configuration of some early octodontoids and putative chinchilloids. The three species co-occur at a single fossil locality and stratigraphic level on the northeastern slope of Barda Negra; P. Sinclairi and D. olsacheri are also recorded in other levels exposed in different isolated outcrops of the Cerro Bandera Formation. Doryperimys olsacheri—like other previously described rodents from this unit— is unknown in other presumably coeval mammal associations, but the biochronological and paleobiogeographic meaning of this assemblage is still under study.