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Terrestrial reproduction presents a thermoregulatory challenge for marine mammals, especially in a context of global warming. Pinnipeds, especially otariids, differ from other marine mammals in that most reproductive processes occur on land. Rocky rookeries rarely provide thermoregulatory resources (shade, pools and wet sand), so pinnipeds reduce thermal stress through thermoregulatory behaviour such as flipper exposure, flipper movement and maintenance of individual distance. Our objective was to analyse climate correlates of thermoregulatory behaviour of Southern sea lions Otaria flavescens in a colony located at the warmest end of its northern distribution on the Atlantic coast of South America. We conducted summer behavioural observations of juveniles/sub-adult (less than 150kg) and adult (300kg) males in the Cabo Polonio rookery, Uruguay. Solar radiation and humidity were positively correlated with thermoregulatory behaviour of sea lions, while ambient temperature had a marginal effect and wind speed had no significant effect. There were no statistically significant differences between age classes in thermoregulation activity. These and previous results on thermoregulatory behaviour of pinnipeds open the possibility that pinnipeds can be limited in abundance or distribution if climate change alters solar radiation in terrestrial rookeries during the breeding season.