The thermal tolerance of ectotherms is a critical factor that influences their distribution, physiology, behaviour, and, ultimately, survival. Understanding the factors that shape thermal tolerance in these organisms is, therefore, of great importance for predicting their responses to forecasted climate warming. Here, we investigated the voluntary thermal maximum (VTmax) of nine grassland viper taxa and explored the factors that influence this trait. The small size of these vipers and the open landscape they inhabit render them particularly vulnerable to overheating and dehydration. We found that the VTmax of grassland vipers is influenced by environmental temperature, precipitation, short-wave flux, and individual body size, rather than by phylogenetic relatedness. Vipers living in colder environments exhibited a higher VTmax, contradicting the hypothesis that environmental temperature is positively related to VTmax. Our findings emphasize the importance of considering local to regional adaptations and environmental conditions when studying thermal physiology and the evolution of thermal tolerance in ectotherms.