During the last 100 years the planet has increased its temperature rapidly, this climate change could bring catastrophic effects for organisms and natural systems. One of the most affected groups is the ectotherms, due to their direct dependence on the environmental temperature. In recent years several studies analyse the effects of temperature and the thermal tolerance on several species of ectotherms. However, there are species whose thermal tolerances are still unknown. Such is the case of critically endangered species the Montseny Brook Newt (Calotriton arnoldi), endemic from the Montseny massif in Spain and whose thermal biology is unknown. Its critical situation makes essential to know its tolerance to cooling, warming and thermopreferendum in water environment where the newt lives. Three experimental procedures were conducted from the western and eastern subpopulations of C. arnoldi considering four classes separately (males, females, juveniles and larvae). The results obtained showed that the CTmax of the species exceeded 31°C, with a significant difference between the two populations. We found that the species tolerates well low temperatures (<1°C), because of the genera Calotriton is adapted to lives in cold waters with temperatures below 15º. Although the thermopreference of the species was expected to trend to cold temperatures, some individuals chose relatively high temperatures, obtaining a range of 11.7°C to 21.6°C. The results presented in this study are an advance in the knowledge of the thermal physiology of this species and support the importance of the temperature of the torrent on its survival. Knowing their thermal limits and their preferred temperature range will help to propose management measures that promote the conservation of streams and its riparian forest cover to mitigate the temperature increase due to climate change.