The lack of numerical age dates for upper Cenozoic strata of the Amazon Basin has prevented resolution of its geologic history and accurate dating of important paleofaunas. Here we present results of magnetostratigraphy and 40Ar/39Ar dating of two volcanic ash deposits from the Madre de Dios Formation of eastern Peru. The two ash ages, 9.01 ± 0.28 Ma and 3.12 ± 0.02 Ma, provide the first numerical age data necessary for accurate interpretation of late Tertiary sedimentation in Amazonia and establish approximate time constraints for the last major cycle of Cenozoic deposition within the southwestern Amazon Basin. The older ash age also provides a minimum age for numerous Amazonian paleofaunas, which allows a more definitive correlation of these paleofaunas with those in other regions of South America.