Calibration of models is a common procedure required to properly reproduce a system under analysis. In the present study, we developed a tool integrating the HEC-RAS NSM-I water quality model with the optimization algorithm SCE-UA in order to allow the automatic calibration.The tool's performance was assessed in two experiments involving a theoretical situation, when it was used to approximate a previously known set of parameters, and a real world application, when the tool was employed to calibrate a water quality model of the Gravataí River, Brazil. The results showed a good capacity to approximate the true parameters values in the theoretical experiment and a good consistency and agreement with reference values in the second experiment. Although future testing and improvements are possible, the proposed approach can be an useful alternative in water quality modeling using HEC-RAS.