Heat stress is known to influence sow reproduction, but it is unknown to which extent under temperate climates. Therefore, the impact of temperature and temperature‐humidity index (THI) on the reproductive performance of sows during summer months from 2009 to 2011 were studied. Reproductive parameters of 22 264 inseminations and 21 610 resulting farrowings of 22 German commercial farms were analyzed. Climatic records were available from the nearest weather station. Effects of temperature and THI at different periods of the reproductive cycle were estimated from May to September where temperatures were expected to influence climatic conditions indoors. High temperatures and THI values 5 days pre‐ and 14 days post‐breeding reduced litter size by 0.01 to 0.03 piglets (P < 0.05). Prior to farrowing, the number of liveborn piglets was reduced (P < 0.05) and of stillborns increased with rising climatic variables (P < 0.001). On the day of farrowing, reduced numbers of weaned piglets were observed when temperature and THI values were elevated (P < 0.05), but pre‐weaning mortality was not influenced (P > 0.05). In conclusion, even under temperate conditions adverse effects of climatic variables on the reproductive performance of sows are present. Thereby, the number of liveborn piglets can be considered as the most sensitive parameter to heat stress. Considering the impact of climate change and predicted heat waves, strategies to mitigate the impact of heat stress are necessary.