Body temperature of butterflies mainly depends on abiotic factors and is relevant for metabolic processes, flight behaviour, and the distribution in various altitudes. We developed a technique for contactless measurement of body temperature of butterflies in the field using a modified infrared thermometer. We caught 522 butterflies from 31 species in lowland and alpine habitats of Austria during flight, and their thorax temperature was immediately measured. The thoracic temperatures ranged from 14.5 °C in alpine Satyrinae species to 39.9 °C in a lowland species. Body temperatures (mean 28.9 ± 6.1 °C) showed highly significant, positive correlation (r = 0.89, p < 0.001) with air temperatures (mean 21.9 ± 7.6 °C). Small alpine species had lowest, but highly variable thorax temperatures while butterflies of lowland areas had higher and lower variable temperature. The mean thoracic temperature was 7.0 ± 3.5 °C higher than air temperature with a maximal difference of 20.2 °C in analpine species. In hot lowland areas some butterflies had a body temperature of up to 3 °C below air temperature. Pieris rapae were active in all habitats showing body temperature tolerance from 34.2 °C in lowland to only 20.8 °C in the alpine area. These data allow conclusions on upper and lower temperature limits for flight. The applied new non-invasive method turned out to be a simple and reliable technique to measure body temperature of butterflies in the field that opens new opportunities for understanding species-specific natural histories.