Understanding the impact of climate change on watersheds using hydrologic models is timely and vital to dam management. The study predicts changes in the inflow of the Magat reservoir using the Soil and Water Assessment Tool (SWAT) under the two Representative Concentration Pathway (RCP) scenarios for future centuries. The monthly calibration process (18 years) and validation process (10 years) of the model resulted in an NSE of 0.73, R2 of 0.74, RSR of 0.52, and PBIAS of 8.38 and NSE of 0.56, R2 of 0.62, RSR of 0.66, and a PBIAS of 17.3, respectively. Under RCP 4.5 and RCP 8.5 scenarios, the model predicted that during the dry and normal years, there will be an average decrease of inflow by 18.56 and 5.41% but an increase of 19.25% during the wet years. Peak flow will likely occur in September for all the scenarios, with a maximum discharge of up to 342.46 m3/s. The study recommends the integration of the model results to update the dam discharge protocol on the forecasting of monthly and annual inflows of the Magat dam to aid the dam management in observing long-term changes in the flow of water going into the reservoir.