Increased ambient temperatures has been identified to contribute to reproductive outcomes for several domesticated species, but its impact on equine reproduction has not been previously investigated. Therefore, this study aimed to examine the relationship between ambient climatic conditions, as measured by temperature-humidity index (THI) between day 7 and day 14 of gestation, and early embryonic loss (EEL) in recipient mares undergoing embryo transfer. The study examined records from 834 embryo transfers at an equine breeding facility in Victoria, Australia. Early embryonic loss was defined as a negative transrectal ultrasound on day 14 of gestation after an embryo was transferred on day 7. Both maximum THI on the day of transfer (day 7) and mean THI between day 7 and day 14 were investigated for association with the outcome of EEL using multivariable logistic regression, controlling for confounders including embryo age and quality, recipient mare quality and embryo transfer quality. EEL was observed in 21% of embryo transfers. A five-unit increase in maximum THI on the day of transfer was associated with an 18% increase in the odds of EEL (p = 0.01). Similarly, the odds of EEL increased by 25% for each five-unit increase in mean THI between day 7 and 14 (p = 0.003). As both single and cumulative episodes of high THI were associated with increased EEL in embryo transfer mares from this equine breeding facility, further studies are warranted to identify similar effects in a broader population, establish causality and evaluate possible mitigation strategies in anticipation of heat waves. •Heat stress reduces reproductive outcomes in mares.•High temperature at equine embryo transfer associated with early embryonic loss.•High temperature after embryo transfer associated with early embryonic loss.