The Deseadan South American Land MammalAge (late Early Oligocene – Late Oligocene) attests to atime of great diversification in the caviomorph rodent fossilrecord. Nevertheless, Deseadan rodent-bearing localitiesin Neotropical lowlands are few and poorly known. Herewe describe the rodent assemblages from two Late Oligocenelocalities, near Contamana, Loreto, Peru. Seven taxaare new to science: Palaeosteiromys amazonensis gen. et sp.nov., Plesiosteiromys newelli gen. et sp. nov., Loretomysminutus gen. et sp. nov., Scleromys praecursor sp. nov,Ucayalimys crassidens gen. et sp. nov., Chambiramys sylvaticusgen. et sp. nov. and Chambiramys shipiborum gen.et sp. nov. These rodent faunas show that caviomorphswere relatively diverse in Peruvian Amazonia during theLate Oligocene, with the co-occurrence of at least threeextant superfamilies: Erethizontoidea, Octodontoidea andChinchilloidea. Additionally, they mark the earliest knownoccurrences of Scleromys, of a small erethizontid closelyrelated to Microsteiromys and of an adelphomyine closelyreminiscent of Ricardomys (all taxa previously restricted toMiocene localities thus far). They also document a formpotentially related to Eosallamys (previously known fromaround the Eocene–Oligocene transition at Santa Rosa inPeruvian Amazonia). Finally, the geographical range ofAdelphomyinae and of Deseadomys is widely expanded tothe lower latitudes of South America for the Deseadaninterval. The latter elements, in addition to the record ofa very primitive species of Scleromys, suggest the absenceof palaeogeographical and palaeoenvironmental barrierswithin the southern cone of South America before theOligocene–Miocene transition.