It is important to examine what future hydrological changes could occur as a result of climate change. In this study, we projected hydrological changes and their consistency under near-future and end-of-21st-century climate in the Chao Phraya River Basin. Through hydrological simulations using output from six AOGCMs under the RCP 4.5 and 8.5 scenarios, we have reached the following conclusions. Our results demonstrate a projected increase in mid-rainy season precipitation under future climate, which is a necessary condition for a large volume of runoff to occur in the late rainy season. Under end-of-21st-century climate, all simulations using six AOGCMs showed a large increase (> 20%) in runoff in Nakhon Sawan catchment under both RCP scenarios. Compared to the capacities of the Bhumibol and Sirikit dams, projected increases in runoff at the end of the 21st century are high. New flood management and mitigation plans will likely be necessary. Ensemble mean increases in precipitation and runoff were higher under RCP 8.5 than under the RCP 4.5 scenario in both projected periods. Thus, higher global mean temperature would cause higher precipitation and runoff in the basin. This inference is also supported by the higher precipitation and runoff projected under the late future compared with under the near-future climate.