Currently, marsupials (modern members of Metatheria), are widely distributed in tropical and subtropical areas of South America, but poorly represented in their fossil record. Except for the species-rich fossiliferous localities of La Venta (Colombia), additional Miocene metatherians from tropical-equatorial South America are sparsely reported in Bolivia, Brazil, Peru, and Venezuela. Here, we introduce new metatherian remains recovered in the late middle Miocene TAR-31 locality, Peru, assigned to the early Laventan South American Land Mammal Age (~13 Ma) by mammalian biostratigraphy. U-Pb analysis made on detrital zircon grains from TAR-31 gave a maximum depositional age of 17.4 ± 0.12 Ma. Three metatherian taxa are recognized at TAR-31: the didelphid Thylamys cf. T. colombianus, plus the paucituberculatans Palaeothentes sp. (palaeothentid) and the new abderitid Pitheculites ipururensis nov. sp. This assemblage, along with a cebid primate at TAR-31, suggests predominantly humid and warm tropical conditions, with the occurrence of both forests and drier habitats in the surroundings. Comparisons with other Miocene metatherian assemblages at low and mid latitudes of South America clearly confirm closest relationships between TAR-31 and La Venta (previously observed on monkeys and rodents), as well as, to a lesser extent, with Acre River local fauna and Madre de Dios (MD-67) and in Brazil and Peru, respectively. The current results further the hypothesis of Western Amazonia as a single and consistent biogeographical region for land mammals over middle Miocene times, at the western edge of the Pebas Mega-Wetland System, and highlight the role of the Amazonian region concerning marsupial Neogene radiation.