This study was conducted to determine the effect of methylsulfonylmethane (MSM) on growth performance, immune function, antioxidant capacity, and meat quality in Pekin ducks. A total of 960 female 1-day-old Pekin ducklings (53.3 ± 0.4 g) were randomly allotted to 3 treatments with 8 replicates of 40 birds, based on their body weight (BW). The experiment lasted 6 wks, and dietary treatments included a corn–soybean meal–based diet supplemented with 0%, 0.15%, and 0.3% MSM, that is, CON, MSM1, and MSM2, respectively. Growth performance, serum profiles, and meat quality were determined. During the period of days 22–42, BW gain (BWG) in MSM2 treatment was higher (P < 0.05) and feed-to-gain ratio (F/G) was lower (P < 0.05) than those of CON and MSM1 treatments. BW gain and final BW in MSM2 treatment were increased (P < 0.05) compared with CON and MSM1 treatments during the period of days 1–42. Serum activities of superoxide dismutase and glutathione peroxidase, total antioxidative capacity, and concentrations of interleukin-2 and interleukin-6 were higher (P < 0.05) in MSM2 than in CON treatment. Ducks in the MSM2 treatment group had lower (P < 0.05) serum malondialdehyde, interferon gamma, and tumor necrosis factor-α levels than those in the CON treatment group. The supplementation of MSM increased (P < 0.05) water-holding capacity and redness (a*) and decreased (P < 0.05) values for 2-thiobarbituric acid and drip loss on day 5. Ducks in the MSM2 treatment group had higher (P < 0.05) pH24h than those in the CON treatment group. Taken together, the inclusion of MSM (0.3%) increased final BW and BWG during periods of days 22–42 and days 1–42, reduced feed-to-gain ratio during the period of days 22–42, and resulted in positive effects on immunity, antioxidant capacity, and meat quality.