This paper presents an intelligent control of a ship's roll motion damping system consisting of a pair of active fins driven by a hydraulic system. The suggested intelligent controller updates coefficients of the proportionalderivative- second derivative controller (PDD2) using particle swarm optimization (PSO). The memory-based and the triggered-memory-based PSO algorithms are studied to improve the performance of PSO-PDD2 algorithm for various sea conditions using simulations. The resulting triggeredmemory- based PSO-PDD2 controller shows a superior total roll motion damping performance. Damping of small roll motions has also been achieved. The PSO-PDD2 active fin system controller has been implemented for the sample ship, Marti, in a full-scale sea trial in real time. Results of both simulations and real-time trials are presented. Application of PSO has yielded very promising results not only for PDD2 roll damping controller but also for other possible marine control application.