In this paper, a Lyapunov-based adaptive 2nd-order sliding mode controller is proposed to control the current in an ACTIVE power FILTER (APF). The penetration of APFs has been exponentially increased because of their high flexibility and fewer resonance problems. Moreover, they can compensate high range of current harmonics and reACTIVE power. The voltage and current control loops have always been interesting areas for researchers since the satisfactory performance of the APF is highly dependent on these control loops. A sliding mode controller (SMC) is a mighty controller when uncertain conditions are considered. However, in order to reduce the chattering- high-frequency switching- and improve the steady state operation, stability, and robustness of the controller, it is usually decided to adaptively tune the gains of the controller. In this paper, a simple-structure adaptive SMC (ASMC) is proposed which can be implemented easily. This ASMC is shown to be stable using the Lyapunov theorem and proved with SIMULINK simulation that it has less steady state error, less chattering, and faster dynamic response compared to the conventional SMC.