Body temperature influences the activity and behavior of reptiles, with warmer body temperatures typically being associated with improved performance. Nocturnal ambush-hunting rattlesnakes would therefore benefit from selecting warmer substrate hunting sites, allowing them to stay in ambush longer as the environment cools and, presumably, to have a higher probability of striking prey successfully. Here, we tested whether free-ranging sidewinder rattlesnakes (Crotalus cerastes) might select ambush sites based on substrate temperature. We used a thermal imaging camera to measure snake body temperature and substrate temperature at ambush sites and random locations. Rattlesnake body temperature showed a strong positive correlation with substrate temperature at the ambush site, and the substrate at ambush sites was significantly warmer than randomly chosen sites. This suggests snakes might select ambush sites based on substrate temperature. Desert pit vipers appear highly attuned to environmental conditions and might integrate several criteria to select microhabitats that increase predation success.