An improved adaptive super-twisting sliding mode control is proposed to address the maneuvering control issue for unmanned surface vehicles. Considering the underactuated issue, an output redefinition-based error dynamic system is established. An improved super-twisting sliding mode controller is designed to improve the robustness and avoid the adverse effect caused by chattering, which also improves the transient and steady-state performance of the control system. According to the unknown upper bounds on the derivative of composite uncertainties, the adaptive technique is utilized to estimate control parameters. The stability of the closed-loop system is proved by the Lyapunov theory. Additionally, the performance of maneuvering is verified by numerical simulations. The simulation results show that the proposed controller has satisfied control performance and reduces chattering.