This paper presents a novel controller based on the delayed feedback control theory for stabilizing unstable torsional oscillations caused by Subsynchronous Resonance (SSR) in power systems. The first system of the IEEE Second Benchmark Model which consists of a synchronous generator connected to an infinite busbar through two parallel transmission lines, one of which is equipped with a series capacitor, is used to evaluate the controller's effectiveness. Dynamics of the generator damper windings and the Automatic Voltage Regulator (AVR) are included in the nonlinear model. The controller uses the synchronous generator rotor angular speed as the only input. The difference between the input signal's value Mime in the past and its current value is multiplied with a gain to obtain the controller output, which is then combined into the AVR as the stabilizing signal. The effectiveness of the proposed controller in damping the torsional oscillations is demonstrated via time-domain simulations in MATIAB-Simulink. The optimum values of the controller parameters for various series compensation levels are also investigated.