This paper is concerned with the control of underactuated systems with external disturbances. Using the global sliding mode control (GSMC) technique, a new robust controller is presented to improve the robustness and stability of the system. The conditions of asymptotic stability are presented by linear matrix inequalities (LMIs). Our purpose is to build a control law so that it would enforce the states of the system to exponentially verge the sliding surface. The suggested controller has a simple structure because it is derived from the associated first-order differential equation and is able to handle the external disturbances and system nonlinearities. The efficiency of the proposed scheme is observed through simulations in an illustrative example. Simulation results demonstrate the considerable performance of the suggested technique.