Abstract: The closure of the Indonesian Seaway played a key role in the evolution of the Indo-Pacific Warm Pool and associated atmospheric circulation during the Cenozoic. However, the relationship between the closure of the seaway, the evolution of the warm pool, and the shift in atmospheric circulation remains unclear due to poor constraints in tectonic and paleoenvironmental reconstructions. This study reviews the historical literature, including evidence from pollen records, coal deposits, shallow marine carbonate deposits, and biogeographic evolution. The results show that the hydroclimate in Southeast Asia underwent significant changes during the Oligo-Miocene transition, shifting from relatively dry conditions in the Oligocene to persistently wet conditions throughout the Miocene. Combined with recent simulation studies, it was concluded that the hydrological changes in Southeast Asia were influenced by both global and regional factors. The narrowing and closure of the seaway may have increased the gradient between the east-west thermocline depth and the east-west sea surface temperature in the Pacific Ocean, limiting the exchange of subsurface water between the Pacific and Indian Oceans. This in turn led to a strengthening of the Walker Circulation, which subsequently induced hydrological changes in Southeast Asia after the Oligo-Miocene boundary and mitigated the effects of global cooling over the Late Miocene. Uncertainties remain in current studies, and more geological records and simulation studies in the future would help to accurately characterize the relationship between seaway closure, warm pool evolution, and atmospheric circulation in the Oligo–Miocene.