In this paper, a fractional order sliding mode controller (FOSMC) is proposed for nonlinear coupled tank system. The proposed controller integrates the advantages of the fractional order control and sliding mode control. Fractional order controller has an extra flexibility to meet the specifications of the controller design and sliding mode control is a robust controller, which is able to respondtoexternal disturbances combined with nonlinear systems.The objective of the controller is to allow the system states to move to the sliding surface and remain on it so as to ensure the asymptotic stability of the closed-loop system. The stability analysis for the proposed controller is studied based on the Lyapunov stability theorem. Simulation results show the ability of the proposed controller to improve the system performance especially when the controlled system is exposed to external disturbances and variation of reference trajectory compared with the conventional proportional-integral-derivative controller (PID) and conventional sliding mode controller (SMC), which is based on integer order derivatives.