The current study was carried out to assess the effect of the Rosemarinus officinalis extract (RE) as a feed additive on growth performance, immune response, antioxidant status, and disease resistance of the Nile tilapia (Oreochromis niloticus). Fish were randomly distributed into four groups. G1 served as a control, while G2, G3, and G4 were fed the basal diet supplemented with 0.5%, 1%, and 1.5% of RE, respectively. After the experimental period (8 weeks), fish were experimentally infected with Staphylococcus aureus. The results displayed enhanced growth performance [final body weight (FBW), weight gain (WG), weight gain% (WG%), and specific growth rate (SGR)] with a significant reduction in feed conversion rate (FCR) inG3 and G4. A significant increase of the biometric indices [hebatosomatic index (HIS), splenosomatic index (SSI), and condition factor (K)] was recorded inG3, followed by G4 and G2 compared to G1. Serum levels of glucose, triglyceride, cholesterol, creatinine and nitric oxide (NO) and activities of alkaline phosphatase (ALP) and alanine aminotransferase (ALT) reduced significantly with increasing the concentration of RE fed. While levels of total protein, albumin, total globulin, γ globulin, superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and resistance against S.Aureus were significantly increased in a dose-dependent manner of RE fed. Additionally, the serum activity of lysozyme and level of myeloperoxidase (MPO) increased significantly in treated groups in a descending manner with increasing the concentration of RE in the feed. Thus, in the present work, the addition of ethanolic RE to the Nile tilapia diets proved to enhance growth performance, innate immunity, and physiological status of fish hence, presenting a promising feed additive in aquaculture.