Based on the equations of mo mentum and continuity, a general three-dimensional transport model for suspended sediment by waves and currents is developed by respectively decomposing the instantaneous velocities and concentrations into three different time-scale components. The model is composed of two submode ls. One is a three-dimensional flow field submodel including the influence of waves, and the other is a three-dimensional diffusion submodel for suspended sediment due to waves and currents. The purpose of this study is to provide a theoretical model for further investigating sediment transport by waves and currents in estuarial and coastal areas.