This study conducted to assess the impacts of climate changes on hydrological regime of Langat River Basin using the Soil and Water Assessment Tool (SWAT). The Statistical Downscaling Model (SDSM) applied as a practical tool to bridge the spatial difference between GCM grid-box scale and sub-grid box scale in the Langat River Basin as a tropical area. The calibrated and validated SDSM was applied to project the possible scenarios in two future periods (2030s and 2080s) of meteorological variables. The SDSM predicts increasing mean monthly precipitation and maximum temperature during the two future periods. The Sequential Uncertainty Fitting (SUFI-2) algorithm was used for sensitivity analysis and calibration of the SWAT model in simulating of observed average monthly streamflow. The observed data matched reasonably well with the simulated flows with respect to Nash-Sutcliffe coefficient, PBIAS, RSR, and R² with values of 0.62, 5.7, 0.61, 0.63 during calibration process (1997–2004) and 0.55, 3.5, 0.67, 0.56 during validation process (2005-2011), respectively. The projected results of monthly streamflow revealed that the average annual streamflow may increase about 71% and 108% in future mid-century and significantly for end century period, respectively along with increase in surface runoff.