Storm-driven overwash is a sediment-transport process fundamental to the evolution of low-lying coastal environments. Physical insight into overwash morphodynamics is crucial for improved risk assessment and hazard forecasting in vulnerable coastal zones. Spatially extended observations of washover deposits have shown that back-barrier shoreline planforms can be quasi-periodic. These rhythmic patterns have been attributed to the influence of a forcing template in bathymetry or topography, or as inherent in the forcing itself. With an alternative to this prevailing explanation, we present results of a physical experiment and numerical model in which quasi-periodic patterns in washover deposits are self-organized, arising from interactions between barrier topography, routing of overwash flow, and sediment flux.