In the present study, the efficacy of Verbascum speciosum on growth performance, gastrointestinal structure, immune and biochemical parameters in rainbow trout were evaluated. Verbascum speciosum was supplemented at 40 (low dose group), 250 (medium dose group), and 500 (high dose group) mg kg -1 diet and fed to fish (~9.7 g initially) for 55 days. Verbascum speciosum supplementation significantly enhanced the growth performance in low dose group compared to control group. Serum complement, total antibody, haemagglutination titer and lysozyme activities were significantly higher in fish administered with medium dose diet in comparison with the basal diet. The low dose group showed an increase in the peroxidase level compared with the control group. In low and medium dose groups significantly higher total protein and globulin contents were also seen. Higher alkaline phosphatase (ALP) and aspartate aminotransferase (AST) activities were also observed in fish that received the high dose of Verbascum speciosum. Meanwhile, higher antioxidant activity and lower lipid peroxidation product, indicated by malondialdehyde (MDA), were seen in low dose group. Present study suggests that Verbascum speciosum, especially in low and medium doses of administration could affect growth performance, immunological and biochemical parameters in rainbow trout.