This paper is concerned with delay-dependent robust H∞ control for an offshore steel jacket platform subject to wave force. By taking into account the variations of the structural natural frequency and damping ratio, and time-varying control delay, an uncertain time-delay dynamic modal of the offshore platform is established first. Then a delay-dependent robust H∞ control scheme is proposed to attenuate the wave-induced vibration of the platform. A delay-dependent robust H∞ controller is obtained in terms of the solvability of a nonlinear minimization problem. Simulation results show that though there exist timevarying control delay and parameter uncertainties in the system, the designed controller can effectively attenuate the vibration of the offshore platform and keep the stability of the system.