The interactions of porous vertical-cylinder array with incident waves and its performance as a breakwater are investigated by numerical simulations and medium-scale experiments. The interaction theory based on linear potential theory for N bottom-mounted vertical cylinders and the porous and viscous effects are included through Darcy's law. The empirical relationship between plate porosity and porous parameter is obtained from a series of experiments. The numerical results agree well with the measured data. It is found that both transmitted and reflected waves as well as wave run-up and exciting forces are significantly reduced due to wall porosity. The results also show that the optimal design of seawater-exchanging breakwaters can be found through a systematic parametric investigation by controlling the size, number, porosity, and arrangement of circular cylinders. Copyright © 2009 by The International Society of Offshore and Polar Engineers (ISOPE).