The feeding experiment's main purpose was to estimate the influence of fortification of tilapia diets with miswak leaf powder (MLP) on performance, digestive enzyme activity, chemical body composition, antioxidant status, immune response, and resistance against bacterial and fungal infections. O. niloticus fingerlings (15.22 ± 0.06 g) were fed four formulated diets fortified with MLP at 0, 2.5, 5, and 10 g kg−1 diet for 56 days. The fish were injected with Aeromonas hydrophila for 14 days at the end of the feeding trial, and then the mortality percentage was recorded daily in all treated groups. The obtained results showed that dietary administration of MLP at 2.5 g kg−1 recorded significantly higher values in all growth, feed utilization, and feed efficiency parameters. The digestive enzyme activity was significantly (P < 0.01) higher in fish fed a low level of MLP (2.5 or 5 g kg−1) when compared with the control group. Furthermore, the higher concentration of MLP (10 g kg−1) significantly reduced the mortality rate induced by Aeromonas hydrophila infection, increased crude protein percentage in fish body and represented highly in vitro antibacterial and antifungal efficiency. Based on the findings of the challenge tests against A. hydrophilia, higher survival was found in infected fish, explaining that MLP at high level of 10 g kg can be used to resist the outbreak of the A. hydophilia disease in tilapia. In conclusion, our obtained data showed that applied MLP in tilapia diets could promote performance, feed efficiency, antioxidant status, immune activity, and enhanced resistance of O. niloticus against infections.