Animals that live in environments where temperatures approach physiological tolerances need to thermoregulate effectively to minimise exposure to potentially lethal or sub-lethal temperatures. The capacity for an animal to thermoregulate, however, can be restricted by competition from other animals. Sandstone outcrops are thermally extreme environments, especially in summer. For nocturnal, rock dwelling animals on sandstone outcrops, diurnal thermoregulation is limited to the selection of thermally suitable retreat rocks. As rocks are limited in number, there should be competition for quality retreat rocks. We used the flat rocks spider, Morebilus plagusius, a nocturnal, sit and wait predator on sandstone outcrops to determine if spiders use microhabitat selection as a form of behavioural thermoregulation for avoiding temperatures close to their thermal tolerances. We also determined if ontogenetic competition restricts behavioural thermoregulation. We found that although M. plagusius has high thermal tolerances which acclimatise seasonally, adult spiders use rocks non-randomly which decreases their exposure to extreme temperatures. Juvenile spiders, however, use hotter more thermally variable rocks as retreats. We used behavioural trials, to show that spiders compete intraspecifically for thermally suitable rocks, and that larger spiders outcompete smaller spiders for higher quality thermal retreats. Our results demonstrate that thermoregulation is often restricted by biotic interactions, such as, intraspecific competition. Our research highlights the importance of integrating ecological, physiological and behavioural components when understanding how animals thermoregulate in stressful thermal environments.