A study was conducted to evaluate the effects of chestnut tannins (CT) on the meat quality, welfare and antioxidant status of heat-stressed lambs. Lambs in one group were raised at 20°C and fed a basal diet (N), and three other groups (32°C) were fed a basal diet with 0 (CT0), 5 (CT5), and 10 g (CT10) of CT/kg. Addition of CT increased the b* and L* values of meat and superoxide dismutase and glutathione peroxidase activity in the serum and liver of heat-stressed lambs. The malondialdehyde concentration in meat, serum, and liver of heat-stressed lambs was decreased by dietary CT supplementation. Lambs in the CT0 group had higher cortisol, T3, and T4 levels, creatine kinase activity, white blood cell count, neutrophil count, neutrophil:lymphocyte ratio and a lower lymphocyte count than that in the N and CT10 groups. In conclusion, the addition of CT improved meat quality, certain stress parameters, and the antioxidant status of heat-stressed lambs.