Abstract It is significant to reduce the wave‐induced vibration and ensure the safety of the offshore structure. This paper deals with the observer‐based sliding mode vibration control problem of offshore structures with nonlinear energy sink (NES) mechanisms. By considering the coupled nonlinear characteristics of unmodelled dynamics, system uncertainty, and external wave force, a more general nonlinear system model of the offshore structure equipped with an NES mechanism is presented. Then, an observer‐based sliding mode control scheme for the nonlinear structure‐NES system is developed. The existence condition and the design algorithm of the observer‐based sliding mode controller are provided. Simulation results show that the observer‐based sliding mode controller is capable of reducing the wave‐induced vibration amplitude of offshore structures effectively. Compared with the NES‐based passive control, the observer‐based sliding mode controller can further reduce the response amplitudes of the offshore structure remarkably. In addition, the designed observer‐based sliding mode controller is robust the variations of structure parameters and the external wave loads, thereby presenting potential advantages of the active damping control of the offshore structure.