An experiment was conducted to investigate the effects of Aegle marmelos fruit (AMF) extract on the growth performance, biochemical parameters, immune response, antioxidative capacity, and digestive enzyme activity of Nile tilapia (Oreochromis niloticus). Fish were fed a diet supplemented with AMF at concentrations of 0 (AMF0; control), 5 (AMF5), 10 (AMF10), 15 (AMF15), or 20 (AMF20) g/kg for 8 weeks. The results show that the final body weight, weight gain, specific growth rate, average daily gain, and feed conversion ratio were significantly higher in fish fed AMF15 and AMF20 compared to those fed the control diet (P < 0.05). Moreover, significant increases in antioxidant enzyme activities and non-specific immune responses were observed in groups fed AMF15 and AMF20. Interestingly, the level of cholesterol decreased with increasing AMF concentrations in the diet. As dietary AMF levels increased, digestive enzyme activities significantly improved. After the feeding trial, fish were injected intraperitoneally with Streptococcus agalactiae, and the 14-day cumulative mortality was calculated. A high survival rate after challenge with S. agalactiae was observed in all groups that received AMF-supplemented feed. Therefore, the present study suggests that supplementing the diet of Nile tilapia with AMF at a concentration of 20 g/kg could encourage their growth, improve their immunity and antioxidant status, and provide strong protection against S. agalactiae.