The Asian sea bass is one of the most economically important species in aquaculture, and improving its health and production performance requires the use of novel nutritional additives. Accordingly, the present study was designed to investigate the effects of methylsulfonylmethane on growth performance, survival, immunity, antioxidant activities, and the expression of IGF-1, IL-1β, Lysozyme, SOD, and HSP70 genes in Asian sea bass juveniles. In this experiment, 300 juvenile fish with an average initial weight of 0.98 ± 0.08 g and an average length of 3.84 cm were randomly distributed into 15 tanks and fed for 42 days with five dietary treatments, including a control group and diets containing 0.5, 1, 2, and 4 g MSM per kilogram of feed, each with three replicates. At the end of the trial, the highest final mean weight (17.83 ± 0.335 g) and length (11.19 ± 0.224 cm) were observed in fish fed with 4 g MSM per kg diet, while the lowest final mean weight (14.67 ± 1.056 g) and length (10.57 ± 0.155 cm) were recorded in the 0.5 g treatment. The results showed that MSM had no significant effect on growth performance or survival rate among the treatments (P > 0.05). Among the immune indices, the AST enzyme level in the 0.5 g MSM treatment and the ALT and ALP levels in the 0.5 and 4 g treatments were significantly lower than those in other treatments (P < 0.05), indicating that the 0.5 g/kg dose was optimal for improving immune parameters. The highest superoxide dismutase (SOD) activity was observed in the 1 and 2 g treatments, while the highest levels of glutathione peroxidase (GPx), catalase (CAT), and total antioxidant capacity (TAC) were recorded in the 2 g treatment - all showing significant increases compared to other treatments (P < 0.05). Therefore, 2 g/kg was identified as the optimal level for enhancing antioxidant status. Gene expression analysis revealed that IGF-1 and lysozyme expression levels were significantly upregulated in the 2 g MSM group, while IL-1β and