An optimal active sliding mode controller with specified decay rate design is propesed to control the wave-induced offshore jacket-type platform with active mass damper (AMD). Irregular wave loading is approximated by finite sums of Fourier series, and its dynamic characteristics are governed by an exosystem derived from the linearized Morison equa-tion. The offshore platform system is first decomposed into two virtual subsystems based on a linear transformation. By considering the velocity of AMD as a virtual control force for the first subsystem, the optimal virtual controller is derived. Furthermore, an optimal sliding mode surface with specified decay rate is proposed. Then, the active sliding mode con-troller is designed to ensure that the state trajectories reach the sliding surface in finite time and remain on it thereafter. Numerical simulation is employed to verify the effectiveness of the proposed approach.