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) pH
24h
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.