Due to the significant increase in the human population, satisfying its energy demand becomes a major challenge at present and in the future. Hydropower is a form of renewable energy that converts the potential or kinetic energy of water to useful mechanical power. Considering the low speed of streaming water, hydraulic Savonius turbines can be effectively used for such conditions. Such turbines are robust and very simple to manufacture. However, Savonius turbines suffer from a poor efficiency. For this purpose, an optimization process was done to improve the power density of a hydraulic Savonius turbine while keeping its simple structure. Here in this study, the two main geometry parameters, i.e., 1) the overlap ratio, 2) the gap ratio, are served. Furthermore, the optimal operating range for both parameters is presented. To achieve this goal, many different geometries of the turbine are simulated by using the computational fluid dynamics (CFD) code Star-CCM+. In the optimization process, all simulations are driven by the in-house software OPAL++. The output power coefficient (Cp) is considered as a single target function to be maximized. The performance of the optimal geometry is compared with the standard design over the whole range of operation, leading to an improvement in the performance of the turbine by about 8%.