Robust trajectory tracking is a vitally important issue to underactuated unmanned surface vessels (USVs). In this paper, a sliding-mode control method with switching-gain adaptation is proposed to force an underactuated USV to track a predefined trajectory, despite the presence of parametric uncertainties and unknown environmental disturbances induced by the wave loads, the wind and the ocean currents. The proposed controller is designed using sliding-mode control (SMC) and backstepping and switching-gain adaptation techniques. In this methodology, SMC and backstepping technique are combined to enhance the robustness of the system, and the switching-gain adaptation law based on the ideal switching-gain is employed to alleviate the chattering problem. In addition, we have proved the global asymptotic stability of the closed-loop system under the discontinuous thruster forces or the varying uncertain terms related to the velocities and the environmental disturbances. Finally, numerical simulations are provided to demonstrate the robustness and effectiveness of the proposed method for trajectory tracking of underactuated USVs.