While the wave attenuation capability of mangroves has been extensively studied, limited research has addressed the subsequent wave–dike interactions after waves propagate through mangrove forests. This study develops and validates a numerical model to simulate these interactions under extreme wave conditions. In the simulation, mangrove canopies and the revetment of the ecological dike are treated as porous media, whereas the dike core is considered impermeable. The model is used to investigate the influence of mangroves on wave transformation and the resulting dike overtopping. Seven mangrove configurations and two belt lengths are tested, including placements near the dike toe, on the slope, and on the seabed. Results show that mangroves near the dike toe effectively reduce wave height but also induce significant wave setup. While the reduction in wave height mitigates overtopping, the wave setup exacerbates it, resulting in an overall overtopping discharge reduction that reflects the combined effects. When the mangroves are near the dike toe, the relative overtopping rate \(q/{q}_{base}\) ranges from 2% to 77%. Extending the mangrove belt may further mitigate overtopping, but its engineering feasibility requires experimental and economic validation.