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Simple Summary: The ability of animals to maintain their body temperature within an optimal range, known as thermoregulation, is essential for their survival, overall health, and daily activities. Ectotherms, including reptiles, rely on external energy resources to regulate their body temperature. How well they can achieve this, heavily depends on various environmental factors, such as the climate and its seasonal changes. Islands typically have a mild climate, which is expected to favor the thermoregulation of reptiles throughout the year. In this study, we investigate the effect of seasonality on the thermoregulation efficiency and behavior of a population of lizards found on Naxos Island, in the Cyclades, Greece. Our results reveal that seasonal fluctuations significantly influence how easily and precisely lizards can regulate their body temperature, with summer being the most favorable period, and autumn being the least favorable. Interestingly, lizards adjusted their thermal preferences and thermoregulation efficiency depending on the challenges imposed by each season and thus managed to maintain stable body temperatures. Whether these adjustments represent evolutionary adaptations or simply reversible shifts, awaits further research. Understanding how lizards adapt to their changing environment can provide valuable insights into their survival strategies and how they may cope with future environmental changes. Ectotherms, including lizards, rely on behavioral thermoregulation to maintain their body temperature within an optimal range. The benign climate of islands is expected to favor the thermoregulation efficiency of reptiles throughout their activity period. In this study, we investigated the seasonal variation in thermoregulation in an insular population of the roughtail rock agama (Laudakia stellio) on Naxos Island, Greece. We measured body, operative, and preferred temperatures across three seasons (spring, summer, and autumn), and we evaluated the effectiveness of thermoregulation, using the Hertz index (E). Our results revealed that the effectiveness of thermoregulation was significantly influenced by seasonality. E was quite high in summer (0.97) and spring (0.92), and lowest in autumn (0.81). Accordingly, the quality of the thermal environment was significantly low during autumn, and maximum during summer. However, despite the environmental temperature fluctuations, lizards exhibited remarkable stability in body temperatures. They also adjusted their preferred temperatures seasonally and doubled the thermal niche breadth they occupied during summer, thus enhancing thermoregulation efficiency. Whether or not these adjustments are plastic or fixed local adaptations remains to be explored in further research across multiple years and seasons, including additional insular populations. [ABSTRACT FROM AUTHOR]