This study explored the valorization of watermelon outer peel powder (WOPP) as a sustainable functional ingredient for nutritional fortification of wafer snacks. WOPP was characterized by low moisture, high ash, substantial crude fiber, moderate protein and fat, and a rich mineral profile dominated by phosphorus, calcium, potassium, and iron. Notably, WOPP exhibited exceptional antioxidant capacity, with high total phenolic content and strong DPPH radical scavenging activity. Wafers were formulated with 0%, 5%, 10%, and 15% WOPP substitution of wheat flour. Nutritional analysis revealed dose-dependent increases in moisture, ash, fiber, protein, fat, and minerals in fortified wafers, accompanied by a corresponding decrease in available carbohydrates. Antioxidant properties of wafers were markedly enhanced, with the 15% WOPP wafer showing the highest total phenolic content and radical scavenging activity. Sensory evaluation by trained panelists indicated that wafers with 10% WOPP substitution achieved the highest overall acceptability, balancing improved nutritional and antioxidant profiles with favorable taste, texture, and appearance. These findings demonstrate that WOPP is a viable, nutrient-dense, and antioxidant-rich ingredient that can be successfully incorporated into wafer snacks without compromising sensory quality. The study provides a practical model for converting agricultural waste into value-added functional foods, addressing both environmental sustainability and nutritional challenges in snack product development. Future work should focus on scaling production, shelf-life stability, and in vivo nutrient bioavailability assessment.