Abstract: Seawalls are essential coastal engineering structures that block tidal intrusion and mitigate wave overtopping, thereby ensuring flood protection for coastal areas. They play a critical role in safeguarding low-lying regions from storm surges and wave-induced flooding. However, under extreme storm surge conditions, the combined effects of strong winds and waves can significantly increase overtopping discharge, posing serious threats to the safety and stability of coastal defenses. To address this issue, this study developed a numerical model of seawall overtopping under coupled wind–wave conditions using the open-source CFD (Computational Fluid Dynamics) platform OpenFOAM. The model is based on the RANS (Reynolds-Averaged Navier–Stokes) equations, extended to a porous-medium framework through the VARANS (Volume-Averaged RANS) formulation, and incorporates the VOF (Volume of Fluid) method to capture air–water interface evolution. Wind forcing was introduced via both surface shear stress and air-pressure gradients to represent the combined aerodynamic effects of drag and suction at the free surface. The model was validated and calibrated against a series of wind–wave flume experiments conducted in a 60 m long, 0.8 m wide, and 1.8 m high closed wind–wave tank equipped with a blower system capable of generating wind speeds up to 15 m/s. The experiments employed regular waves with a wave height of 0.12 m, a period of 1.8 s, and a still-water depth of 0.52 m. The physical model seawall consisted of a permeable rubble mound armored with Accropode blocks and a parapet at the crest. Measurements of free-surface elevation, overtopping discharge, and wind velocity were used for model validation. The numerical results showed good agreement with the experimental data in terms of wave profiles, free-surface time histories, and overtopping volumes, with relative errors generally below 12%. This confirms the reliability and accuracy of the proposed numerical approach in reproducing the complex wind–wave interaction processes during overtopping. Using the validated model, comparative numerical analyses were conducted for seawalls with different parapet heights to examine variations in overtopping discharge, overtopping layer thickness, and impact forces under both windy and calm conditions. The simulation results revealed that the presence of wind markedly amplifies overtopping discharge and modifies the jet structure. As the parapet height increases, the overtopping jet thickness exhibits a distinct non-monotonic trend—it first increases due to enhanced vertical deflection and energy concentration induced by wind-driven acceleration, and then decreases as the parapet becomes sufficiently high to effectively block and reflect the overtopping flow. Moreover, the peak instantaneous impact force on the parapet under wind forcing was approximately 1.5 to 2 times greater than that under no-wind conditions, consistent with experimental observations. The amplified impact load results from increased jet momentum and turbulent airflow above the crest that intensifies local pressure fluctuations during collision with the parapet. In addition, the simulation results showed that the mean overtopping discharge decreases exponentially with increasing relative freeboard, while wind forcing substantially shifts the reduction curve upward, indicating that even moderate winds can significantly enhance overtopping under certain geometric configurations. The findings suggest that an optimal range of parapet height exists that minimizes overtopping discharge while preventing excessive impulsive forces. This provides valuable guidance for the geometric optimization of parapet structures in practical engineering design. Overall, this study presents a comprehensive framework for simulating wave overtopping under coupled wind–wave conditions and quantitatively evaluating the effects of wind on hydrodynamic and structural responses. The developed VARANS–VOF model can be applied to a broad range of coastal defense structures, including breakwaters and levees, and offers a theoretical basis and technical support for parapet optimization in wind–wave environments. The results contribute to improving the design accuracy of coastal protection systems and enhancing their resilience under future extreme storm and climate-change scenarios.