ABSTRACTAn eight-week feeding trial was conducted to evaluate the effect of lemon verbena (Aloysia triphylla) extract on growth performance, some haematological, biochemical, and non-specific immunity parameters as well as stocking density challenge of rainbow trout (Oncorhynchus mykiss). After adaptation, the fish, with an average weight of 25.52 ± 1.31 g, were fed four different dietary lemon verbena inclusion levels (0, 2.5, 5, and 7 g/kg) for six weeks. In the next step, the number of fish in each tank tripled to expose the fish to density challenges for two weeks. There were differences in the final weight, (SGR), (FCR), (PER), and feed intake between the experimental and control groups (p < 0.05). The results showed that the cortisol content decreased (p < 0.05). The highest levels of FCR, feed intake, plasma triglyceride, cholesterol, (LDL), and (HDL) were observed in the control group. Furthermore, under high stocking density, treatments showed a significant decrease in glucose and cortisol (p < 0.05). A significant increase was also shown in (IgM), (RBA), and (SOD) in all treatments. Finally, using lemon verbena extract, up to 7 g/kg of food can be recommended for rainbow trout diets because it can improve growth performance and non-specific immunity parameters.