The protective effects of chitosan (CS) supplementations on oxidative stress induced by diquat in weaned piglets were investigated. A total of 36 crossbreed piglets with an average live body weight (BW) of 8.80 ± 0.53 kg were weaned at 28 ± 2 days and randomly divided into six dietary treatments (n = 6): control (basal diet), negative control (10 mg diquat/kg BW injected to piglets fed with basal diet), and basal diet treatments containing either 250, 500, 1000, or 2000 mg/kg of CS administered to piglets injected with 10 mg diquat/kg BW. The experiment conducted for 21 days which consisted of pre-starter period (14 days) and starter period (7 days). BW, feed intake, and fecal consistency were monitored. Blood samples were collected to determine antioxidative and immune parameters. CS supplementation improved the growth performance and decreased fecal score of piglets from days 1 to 14. Diquat also induced oxidative stress and inflammatory responses by decreasing the activities of antioxidant and regulating cytokines. But dietary CS alleviated these negative effects induced by diquat that showed decreasing serum concentrations of pro-inflammatory cytokines but increasing activities of antioxidant enzymes and anti-inflammatory cytokines. Results indicated that CS attenuated the oxidative stress of piglets caused by diquat injection.