Malic acid (MA) is one of the organic acids that has been less studied for its effects on fish growth performance and gut health. The present study aimed to assess the effects of dietary MA supplementation on growth performance, antioxidant and immunological parameters, and intestinal gene expressions in rainbow trout, Oncorhynchus mykiss. Fish with similar to 25 g body weight were randomly distributed into 12 tanks at a density of 15 fish per tank and were fed with diets containing 0 (control), 0.25, 0.5, and 1% MA over 8 weeks. The results indicated that dietary MA persuaded no significant changes in growth performance, feed intake and survival rate. There were elevations in serum alanine aminotransferase (ALT; 0.5 and 1% MA), aspartate aminotransferase (AST; 0.5 and 1% MA), lysozyme (0.25, 0.5 and 1% MA), catalase (CAT; 0.5 and 1% MA), superoxide dismutase (SOD; 1% MA), glutathione pemxidase (GPx; 0.25 and 0.5% MA) activities, and total immunoglobulin (Ig) level (0.5% MA) in the fish MA-supplemented diets, compared to the control. Serum alkaline phosphatase (ALP) activity showed a decrease in 0.5% MA treatment compared to the control. Skin mucus lysozyme (0.25% MA), alternative complement activity (ACH50; 0.5% MA), and total Ig (0.25 and 0.5% MA) showed significant elevations, whereas skin mucus ALP (0.5% MA) exhibited a significant decrease in MA-treated fish, compared to the control fish. Intestinal il-1 beta (0.5 and 1% MA), hsp70 (0.5% MA), tnf-alpha (0.5% MA), tgf-beta (1% MA), caspase 3 (1% MA), and caspase 9 (1% MA) expressions significantly increased in M-treated fish, compared to the control. In conclusion, this study showed that dietary inclusion of malic acid at the rate of 0.25% in O. mykiss diets improved the antioxidant status and immune responses of fish without any negative effect on growth performance.