Rising demand for fish products and the high cost of fish meal (FM) as well as soybean meal (SBM) in aquafeeds highlight the need for alternative protein sources. In Pakistan, watermelon is widely cultivated, and its seeds are commonly discarded. In this study, watermelon seeds were used to prepare fish feed. This experiment comprised six treatments, each with three replicate tanks, containing fifteen fish per tank. Test diets (TDI-TDVI) included different levels of watermelon seed meal (WMSM) as 0%, 20%, 40%, 60%, 80%, and 100%. Fingerlings with an initial average weight of 7.21 g were transferred to tanks with a 70 L water-holding capacity and were fed twice a day for 70 days. The fecal samples were obtained daily for chemical analysis. Fish fed with a 60% WMSM-based diet (TD-IV) achieved the maximum growth indices: final weight (29.27 g), weight gain (21.72 g), weight gain percentage (289%; increase from initial weight), specific growth rate (1.51), and survival rate (98%). However, nutrient digestibility varied: GE peaked at 40% WMSM, while CP and EE digestibility were highest at 60% WMSM. The highest hematological indices (RBC: 3.12×106 mm^-3, Hb: 8.27 g/dL, PLT: 69.38×10³ /µL, Hct: 36.13%) were observed at 60% WMSM, whereas PCV peaked at 40%. From these findings, it was concluded that WMSM can replace soybean meal with an optimal inclusion level between 40% and 60%, depending on the performance parameter considered, without compromising the health of fish.