Social factors have important effects on individuals’ fitness, yet the proximate mechanisms remain virtually unknown. Oxidative stress, which results from the imbalance between the systemic effect of reactive oxygen species on molecules and an organism’s capacity to restore oxidative balance, is one potential mechanism mediating such effects. Although individuals’ oxidative balance seems to be affected by their social status, the effects of social environment, and particularly group size and composition, on oxidative balance have seldom been determined in wild populations. Here, we investigated the influence of group size and composition on the oxida- tive balance of wild dominant Alpine marmots (Marmota marmota). We simultaneously measured an index of circulating oxidative damage to lipids (thiobarbituric acid reactive substances) and specific activity of the main enzymatic antioxidant defense (superoxide dismutase). Our results showed a marked decrease in both oxidative damage and antioxidant activity in individuals from larger groups, highlighting a beneficial effect of group size on individuals’ oxidative balance. We discussed the possibility that these physiological benefits originate from social thermoregulation alleviating the cost of hibernation. This study extends our understanding of the proxi- mate factors underpinning the evolution of sociality and opens up novel perspectives within the field of evolutionary ecology.