<p>The multi-stage seawall is a novel coastal structure designed to protect shoreline landscapes and infrastructure. However, studies on wave interactions with such structures remain limited. In this study, a numerical model based on OpenFOAM® was developed to simulate wave propagation over a multi-stage seawall. The two-phase incompressible Navier-Stokes equations were solved, incorporating the <i>k–ε</i> turbulence closure model and a modified Volume of Fluid (VOF) method for accurate free-surface tracking. The model was validated against both newly acquired laboratory data and published results, showing good agreement in predicted free-surface elevations along the seawall. A series of numerical experiments were conducted to analyze wave evolution and overtopping discharge across the multistage platforms, with particular focus on the interplay between vertical step heights and horizontal platform widths. Based on the results, a modified formula for estimating wave overtopping at multi-stage seawalls was proposed. These findings contribute to a better understanding of hydrodynamic behavior and support improved engineering design of multi-stage seawalls.</p>