In recent years, supplementing herbal medicines to fish diets in the aquaculture industry as a growth stimulant and immunostimulant is a common practice. In the present study, we investigated the effect of hydroalcoholic extract of dill on the growth performance and body composition of Rainbow Trout Oncorhynchus mykiss and its inhibitory effect against Aeromonas hydrophila in vitro. We also evaluated the immune responses and antioxidant system in fish that were fed dill diets over a period of 56 d. For this purpose, 180 Rainbow Trout fingerlings (initial weight 13.30 ± 0.22 g; mean ± SD) were purchased and they were fed four experimental treatments, which included a control diet and diets that were supplemented with 1Dill (1 g/kg dill extract), 1.5Dill (1.5 g/kg dill extract), or 3Dill (3 g/kg dill extract). According to the results, the fish that were fed 1Dill (54.70 g) and 1.5Dill (53.19 g) diets had significantly higher weight gain, although the feed conversion ratio was not significantly different among groups. However, the fish that were fed the 1.5Dill (62.55%) diet had significantly higher protein contents in the body than those that were fed the 3Dill (60.09%) treatment; the fish that were fed the 3Dill (29.00%) diet had higher lipid content than those that were fed the other diets. Regarding immune and antioxidant systems, the Rainbow Trout that were fed the 1.5Dill diet had significantly higher values of lysozyme in their serum (53.33 U/mL), alternative complement pathway hemolytic activity (146.33 U/mL), superoxide dismutase (52.33 U/mg protein), and catalase activity (164.66 U/mg protein) than the others did. Dill extracts in different dosages had no inhibitory effect against A. hydrophila in vitro. In summary, it can be concluded that the 1.5Dill diet can be used as a natural stimulant of growth, immunity, and the antioxidant system in the aquaculture industry.