The Savonius wind turbine is known for its simplicity, outstanding cost-effectiveness and low noise, making it ideal for wind energy harvesting in urban areas. However, its success is hampered by the low coefficient of performance. Several geometrical modifications have been proposed by researchers to improve its performance. The present study aims to investigate the impact of the endplate addition on the coefficient of performance of the Savonius wind turbine. The unsteady Reynolds Average Navier Stokes equations and the turbulence equations corresponding to SST κ-ω were solved using the commercial CFD package ANSYS FLUENT. An endplate-integrated 3D model and an endplate-free 3D model of the same geometrical configuration of 1 m diameter and height were simulated under the condition of tip speed ratio 0.8. The extracted results were compared with the existing experimental and numerical data under the same conditions and a close agreement was reached. The endplate-filled turbine showed a significant increase in performance. The lift and drag coefficients were enhanced by 4 and 1.8 times, respectively. Flow contours of pressure, velocity and turbulence intensity were obtained at an angular rotation of 90° where there was a maximum increase in performance to explain how the endplate brings about the change.