Abstract: The aim of this study was to investigate the effects of melatonin on the growth, antioxidation and nonspecific immunity of Oncorhynchus masou. The experiment randomly divided 450 healthy O.masou with similar initial weights into five groups, with three replicates in each group and 30 fishes in each replicate. Fish were fed diets supplemented with different concentrations of melatonin (0, 30, 60, 90, and 120 mg/kg) for a 56-day trial period. The results showed that the addition of 60, 90 and 120 mg/kg melatonin to the feed significantly increased the average weight gain rate, specific growth rate, and feed efficiency of O.masou . The addition of melatonin at doses of 30, 60, 90 and 120 mg/kg to the feed significantly increased the fat percentage. The feed efficiency of the group with 90 mg/kg melatonin added to the feed were significantly higher than those of the other groups. Compared with the control group, adding 30, 60, 90 and 120 mg/kg melatonin to the feed significantly increased the activities of Total Antioxidant Capacity(T-AOC), Total Superoxide Dismutase(T-SOD), and Glutathione(GSH) in the serum, as well as T-AOC, T-SOD, and Glutathione Peroxidase(GSH-Px) in the liver. The levels of Catalase(CAT), GSH-Px in the serum and GSH, CAT in the liver were significantly higher in the 60 and 120 mg/kg melatonin groups than those in other groups. Adding 30, 60, 90 and 120 mg/kg melatonin to the feed significantly reduced the Malondialdehyde (MDA) content in the serum and liver of O.masou. The activities of T-AOC, T-SOD, GSH, CAT, and GSH-Px were highest in the 90 mg/kg melatonin group, significantly higher than those in other groups. The MDA content in the 90 mg/kg melatonin group was the lowest, significantly lower than those in other groups. Compared with the control group, the addition of 30, 60, 90 and 120 mg/kg melatonin to the feed significantly increased the levels of serum Complement Component 4 (C4), Complement Component 3 (C3), Immunoglobulin M (IgM), Lysozyme (LZM), and Alkaline Phosphatase (AKP) in the liver of O.masou. Adding 60, 90 mg/kg and 120 mg/kg melatonin to the feed significantly increased the levels of C3, LZM, IgM and AKP in the serum and C4 in the liver of O.masou. The serum and liver of C3, C4, IgM, LZM and AKP levels were the highest in 90 mg/kg melatonin treatment group, significantly higher than those in other groups. Under the conditions, a dietary melatonin level of 90–120 mg/kg effectively improved the growth performance, antioxidant capacity and non-specific immunity of O.masou. To achieve maximum weight gain, the optimal dietary melatonin level was estimated to be 91.98 mg/kg. These results indicate that melatonin can be used as a beneficial feed additive to enhance the growth, antioxidant status and immune function of O.masou by improving weight gain rate, specific growth rate, feed efficiency and body fatness.