In this paper, we present the use of a pendulum tuned mass damper (PTMD) to reduce vibrations of offshore platforms and determine its optimal parameters. Unlike a conventional TMD, a PTMD can be easily adjusted as the structural parameters change. The offshore platform is considered as a one-degree-of-freedom system subjected to sinusoidal force excitation. For each given mass ratio, the optimal variables include the length of the PTMD and the damping coefficient of the viscous damper. The GA optimization algorithm is employed in this study. Through calculations for a specific offshore platform, numerical simulation results demonstrate the effectiveness of the pendulum in reducing vibrations. Based on the proposed algorithm, the adjustment of the PTMD length according to changes in structural parameters is also discussed. The study can be extended to evaluate vibration reduction when structures are subjected to wave, wind, current, and earthquake loads.