Abstract <p>An adaptive sliding mode controller (ASMC) is presented that can control plants of the <i>n</i>-th order with completely unknown parameters, while using a varying boundary layer thickness for complete chatter prevention. Adaptation laws are derived for parameter estimation and varying boundary layer thickness adaptation. A Lyapunov stability analysis shows that the parameter adaptation laws drive the combined error for the sliding surface to zero while rejecting bounded disturbances. The boundary layer adaptation law automatically grows the boundary layer thickness in the presence of disturbances and reduces it when the disturbances are not present to always keep the sliding surface error trajectory within the boundary layer, which guarantees chatter prevention. Simulations show the superior performance of the proposed ASMC compared to a non-adaptive sliding mode controller with a fixed boundary layer thickness.</p>