The Río San Pedro Formation, located northwest of Valdivia in south-central Chile (∼39°S), preserves a fossil-rich stratigraphic succession that provides new insights into the sedimentary, floristic, and climatic dynamics of southern South America during the Late Eocene. Integrated stratigraphic, sedimentological, and U–Pb LA-ICP-MS geochronological analyses indicate that the formation was deposited within a meandering fluvial system and constrain its maximum depositional age to ∼39.9 Ma. Leaf-bearing strata accumulated primarily in floodplain settings, as indicated by lithofacies associations. The taphoflora comprises a heterogeneous mix of Gondwanan, Neotropical, Pantropical, and endemic elements, dominated by taxa indicative of tropical and subtropical climates. Leaf physiognomic analyses suggest a mesothermal climate, with a mean annual temperature of ∼14–17°C and a mean annual precipitation of ∼1,200 mm. These estimates are broadly consistent with global Late Eocene climate simulations (HadCM3L model, pCO₂ proxies) and align with contemporaneous Paleogene floras from Patagonia and central Chile. Collectively, our findings underscore the persistence of warm and humid conditions across southern South America during this interval and provide an important baseline for understanding subsequent floristic and climatic transitions.