Several dolphin lineages have independently invaded freshwater systems. Among these, the evolution of the South Asian river dolphin Platanista and its closest relatives (Platanistidae) remains virtually unknown as fossils are scarce. Here, we describe Pebanista yacuruna gen. et sp. nov., a fossil dolphin from the Miocene proto-Amazonia of Peru, recovered in phylogenies as the closest relative of Platanista. Morphological characters such as an elongated rostrum and large supraorbital crests, along with ecological interpretations, indicate that this new taxon was fully adapted to freshwater environments. Pebanista constitutes the largest freshwater odontocete known, with an estimated body length of 3 meters. It highlights the ample resource availability in the region, supporting biotic diversity during the Early to Middle Miocene. The finding of Pebanista in proto-Amazonian layers attests that platanistids ventured into freshwater ecosystems not only in South Asia, but also in South America, long before and independently of modern Amazon River dolphin Inia, during a crucial moment for the evolution of the Amazonian realm.