The objective of the existing trial was to examine the effect of enrichment of tilapia (Oreochromis niloticus) diets with rosemary leaf powder (RLP) on growth, antioxidant defense parameters, immune function indicators, and disease resistance. Nile tilapia fingerlings (10.42 ± 0.04 g) were fed four diets supplemented with RLP at 0, 2.5, 5, and 10 g kg−1 diet for 60 days. At the end of the feeding trial, fish were challenged with Aeromonas hydrophila for 14 days, and the mortality rate was reported and documented daily in all experimental groups. The results revealed that dietary inclusion of RLP at 10 g kg−1 significantly enhanced growth performance parameters, feed intake, and feed efficiency compared with other groups. The total serum protein and albumin concentrations were significantly (P < .01) higher in fish fed a high concentration of RLP when compared with control. Kidney function indicators and liver enzymes showed no significant effect by RLP presence except the AST level altered significantly; it decreased with RLP supplementation (P < .05). The antioxidant status and innate immune system were significantly (P < .01) affected by dietary RLP supplementation. Serum catalase, alternative complement, nitroblue tetrazolium, and lysozyme activities significantly increased in fish fed the10 g RLP kg−1 diet. Moreover, the supplementation level of RLP significantly improved the survival rate and in vitro antibacterial resistance against gram-negative bacteria. In conclusion, our results revealed that dietary RLP supplementation improved growth, feed utilization, antioxidant activity, and immune response and disease resistance of O. niloticus. The recommended concentration of RLP in O. niloticus diets is 10 g kg−1.