This study quantitatively evaluates the effectiveness of reinforced coastal defenses in mitigating the impacts of a potential Nankai–Tonankai Earthquake Tsunami across three highly vulnerable coastal prefectures in Japan: Aichi, Wakayama, and Mie. The analysis incorporates the compounding threat of future sea level rise (SLR). A total of 336 tsunami simulations were conducted, combining three dike performance scenarios—ranging from immediate seismic failure, failure due to tsunami overtopping, to full structural resilience—and two SLR scenarios (+0.00 m and +0.50 m). The results demonstrate that structurally resilient dikes consistently and significantly reduce inundation extent, affected population, and building damage, even under elevated sea level conditions. Although regional differences in topography and existing infrastructure result in variation in damage reduction, the benefits of dike reinforcement are evident across all study areas. Nevertheless, the findings also show that SLR exacerbates tsunami impacts under all protection scenarios, regardless of dike performance. These results underscore the critical need for comprehensive tsunami risk reduction strategies that extend beyond structural reinforcement alone.