AbstractAnthropogenic climate change is thought to present a significant threat to biodiversity, in particular to tropical ectotherms, and the effects of long‐term developmental heat stress on this group have received relatively little research attention. Here, we studied the effects of experimentally raising developmental temperatures on a tropical butterfly. We measured survival, development time, adult body mass and wing size of Heliconius erato demophoon (Linnaeus) (Lepidoptera: Nymphalidae) across three temperature treatments. Egg survival was lower in the hotter treatments, with 84%, 73% and 49% of eggs hatching in the 20–30°C (fluctuating temperature with 12 h at 20°C followed by 12 h at 30°C), 23–33°C and 26–36°C treatments, respectively. Larval survival was three times lower in the 26–36°C treatment (8%) compared with the 20–30°C treatment (26%), and we did not detect differences in pupal survival across treatments due to high mortality in earlier stages. Under a moderately increased temperature at 23–33°C, larvae developed faster and adults had a higher body mass and wing loading, but this was not seen in the hottest treatment (26–36°C). Females were also heavier than males in the 23–33°C treatment, but there was no associated increase in wing size. This may suggest a different developmental response to moderately elevated temperatures between the sexes. In summary, high developmental temperatures are particularly lethal for eggs and less so for larvae and also affect adult morphology. This highlights the importance of understanding the effects of temperature variation across ontogeny in tropical ectotherms.