The main aim of this study is to evaluate the effects of common juniper (Juniperus communis) essential oil (JCEO) on growth, digestive enzymes, biochemical profile, immune and antioxidant system, and liver enzymes in the rainbow trout, Oncorhynchus mykiss. 240 fish (10.63 ± 0.06 g) were fed for 60 days in 12 thanks (20 fish per tank) with experimental diets containing JCEO including: control or T1 (untreated), T2 (fed 0.5% JCEO), T3 (1% JCEO), T4 (2% JCEO). Results revealed an increase in final weight (FW) and weight gain (WG) in all treatments, with a significant difference with the control group (P < 0.05). Feed conversion ratio (FCR) decreased, after addition of JCEO compared to the control group (P < 0.05). Amylase, protease and lipase activity levels increased in all groups (T2, T3 and T4 groups), which were significantly different from the control (P < 0.05). The T2 group had the highest total protein (TP) level in serum compared to the control, and all experimental treatments exhibited a significant difference with the control (P < 0.05). The activity of antioxidant enzymes in serum, superoxide dismutase (SOD) and catalase (CAT) increased compared to the control group, with the highest levels in the T4 group. Inclusion of JCEO to the diet significantly reduced the levels of alkaline phosphatase (ALP) and alanine aminotransferase (ALT) activity in blood compared to the control (P < 0.05). The highest level of serum lysozyme activity and total immunoglobulin (total Ig) were observed in T3 group, which displayed a significant difference with the control (P < 0.05). Serum alternative complement activity (ACH50) increased in the supplemented fish compared to the control (P < 0.05). The fish survival increased after challenge with Yersinia ruckeri in JCEO-supplemented fish. In conclusion, JCEO at dietary level of 1% could improve the nutrient digestion and absorption and provide better growth, as well as reinforce the immune system in rainbow trout.