Phenotypic plasticity plays a central role in determining how organisms respond to environmental change over short timescales. Despite this, we know little about how phenotypic plasticity varies between populations or species. We tested the extent of plasticity in basking behaviour in low- and high-altitude populations of two widespread lowland and two highland species of a cool-climate lizard genus: Niveoscincus. We found evidence of divergence in basking behaviour between populations and species, with highland species and high-altitude populations of all species basking more than the widespread lowland species and low-altitude populations. Furthermore, we found differences in the extent of behavioural plasticity between species. Widespread lowland species altered their basking behaviour depending on basking opportunity whereas the highland species maintained high levels of basking independent of basking opportunity. These differences in basking behaviour were concordant with the differences in body temperature across all populations, species and treatments. Combined, this suggests that divergence in thermoregulatory behaviour and thermophysiology between populations and species may have been facilitated by adaptive behavioural plasticity within populations. We discuss this and the implications of our findings for the ability of these animals to cope with ongoing climate change.