As the world warms, species’ thermal sensitivities and tolerances are being tested. However, the temperatures a terrestrial reptile experiences are not only dependent on climate, they are also influenced by thermoregulatory behaviour. Thermoregulatory behaviour may have an especially strong influence on the hotter temperatures a reptile experiences, because reptiles behaviourally avoid dangerously hot conditions. Is it possible, then, that variation in upper thermal limits (CT_max) of terrestrial reptiles primarily reflects thermoregulatory behaviour, rather than local climate? To address this question, we studied CT_max and thermoregulatory behaviour of rainforest sunskinks (Lampropholis coggeri), testing skinks from different populations and in different seasons. We found that CT_max is highly plastic, and, surprisingly, that skinks from colder sites have higher CT_max than do skinks from hotter sites. Skinks from colder sites also select higher body temperatures in a thermal gradient, and CT_max is correlated with selected body temperature in this species. These results are consistent with the hypothesis that upper thermal limits are more strongly associated with thermoregulatory behaviour than with local climate, and emphasize the importance of considering both physiology and behaviour when assessing how species adjust to different climates.