A series of physical model tests were performed to investigate the wave propagation, pressure, and wave overtopping processes for vertical and sloped seawalls on the reef flat. For both regular and irregular waves, the effects of incident wave height, wave period, relative distance, fore reef slope, and still-water depth on wave propagation, wave pressure distribution, and mean wave overtopping discharge were investigated. The peak pressure increased with the increase in incident wave height and decreased with the increase in relative distance and fore reef slope. The mean wave overtopping discharge over the vertical and sloped seawalls increased with the increase in incident wave height, wave period, and still-water depth, but it was significantly reduced with the increase in relative distance and fore reef slope. Modified empirical formulas are proposed for predicting the wave force and mean wave overtopping discharge over the vertical and sloped seawalls on the reef flat for both regular and irregular waves.