The aim of this study was to evaluate the effects of the antioxidant astaxanthin on the immune response, growth, and reproduction of the crayfish Procambarus clarkii (Girard, 1852) stressed with the toxin microcystin-leucine-arginine (MC-LR). P. clarkii exposed to MC-LR were treated with one of four concentrations of dietary astaxanthin supplement (0, 2.5, 5, or 10 mg/g) for 36 consecutive days in tanks. In crayfish exposed to 0.05 mg/l MC-LR, activity levels of superoxide dismutase and alkaline phosphatase in the hepatopancreas significantly increased (P < 0.05) following dietary supplementation with 10 mg/g astaxanthin. The survival and fecundity of P. clarkii decreased significantly with exposure to 0.01 mg/l MC-LR, but astaxanthin supplementation significantly improved survival (P < 0.05). The average daily oxygen consumption rate of P. clarkii stressed with 0.01 mg/l MC-LR was significantly higher than that of unstressed P. clarkii (P < 0.05); astaxanthin supplementation did not significantly affect the average daily oxygen consumption rate of MC-LR-stressed P. clarkii. In conclusion, the antioxidant astaxanthin was found to be a suitable dietary supplement for P. clarkii as it improved immune activity and alleviated MC-LR-induced oxidative stress.