The present study investigates the effects of a deflector on the self-starting speed and power performance of two-bladed and three-bladed Savonius rotors for hydrokinetic applications under low water flow speed conditions. The rotors were tested inside a closed-circuit wind tunnel both with and without a deflector with air as the flow medium. The air data obtained from the experiment were used to calculate the equivalent water flow speed based on the same Reynolds number. The density and viscosity of both mediums were taken into consideration to ensure that the flows were dynamically similar. It was found that the self-starting speeds of the rotors varied with respect to the initial rotor angle. The 3-bladed rotor had lower self-starting speeds than the 2-bladed rotor due to a smaller blade orientation. With the presence of the deflector positioned upstream of the returning blade, the self-starting speeds of both rotors were reduced for all tested deflector angles. The deflector shielded the returning blade of the rotors from being impinged by an incoming flow, reducing the negative torque that would hinder the rotor's rotational motion. The deflector angle of δ = 90° with respect to the incoming flow direction was found to be the optimal configuration, contributing the largest reduction in the self-starting speeds for all initial rotor angles for both rotors. In terms of rotor power performance, the deflector blocking the incoming freestream flow from impinging on the returning blade was found to significantly improve the coefficient of power of the rotors. The largest improvements of coefficient of power by 84.6% and 227.3% were obtained with the deflector configured δ = 90° for the 2-bladed rotor and the 3-bladed rotor, respectively, and the optimal deflector angle was found to be δ = 90°, regardless of the number of blades. The present study has demonstrated the practicality of using a deflector to improve the self-starting speed capability and power performance of Savonius rotors for hydrokinetic application in rivers with low water flow speeds, particularly the narrow and shallower rivers common in rural areas in developing countries.