The aim of this study is to enhance the performance of a twisted Savonius vertical axis wind turbine. The influences of various twist angles (φ) ranged from 0o to 180o and the overlap ratios (δ) of 0 to 0.3 on the rotor performance have been investigated. Thus, the three-dimensional incompressible unsteady Reynolds-Averaged Navier-Stokes equations in conjunction with RNG k-ε turbulence model was utilised and numerically simulated. The numerical results were validated using the available experimental measurements. The predicted flow fields around the rotor were presented and analysed. Accordingly, results indicated that the Savonius rotor with a twist angle of 45o attained a maximum power coefficient (CP) of 0.194 at a tip speed ratio (λ) of 0.8 and wind velocity of 6 m/s. The maximum value of power coefficient at a twist angle (φ) of 45o increased with reducing the overlap ratio and it reached its maximum value of 0.223 at zero overlap ratio. The performance gain (CP) obtained by using the present optimised twisted Savonius rotor was 28 % compared to the conventional untwisted rotor which has CP=0.174. Furthermore, the developed optimised rotor design significantly improved the static torque coefficient to ease the self-starting ability of the Savonius rotor.