In this study, a new model reference adaptive sliding surface (MRASS) design algorithm is proposed for a general class of nonlinear systems. The proposed MRASS methodology generates a nonlinear sliding surface for a given nonlinear plant dynamics by using a model reference sliding surface. Before the adaptation process, a model reference sliding surface is designed for a reference nonlinear system by using the State Dependent Riccati Equation (SDRE) techniques. The designed sliding surface for the reference nonlinear model is then adapted for a given nonlinear plant dynamics. The sliding mode controller for the nonlinear plant is then designed by using the adaptive sliding surfaces. The proposed method is illustrated with an autopilot design for a missile model.