A well-known effect of sub-lethal temperature exposure in a diversity of species is a decrease in reproductive performance. Although this effect has been particularly emphasized for males or male reproductive functioning, it remains to be firmly demonstrated that the effect of heat on fertility is sex-specific. To contribute to this question, here we examined the impact of sub-lethally high temperature on male and female functions in a simultaneously hermaphroditic snail species, Lymnaea stagnalis. Examining hermaphrodites is useful to evaluate the sex-specific impacts of heat exposure, since they possess male and female functions within a single individual, sharing genetic and environmental factors. Moreover, previously developed sex allocation theory allows us to compare the differential performance of sex functions. In this study, we exposed snails to temperatures ranging from 20 to 28 °C for 14 days and assessed their egg and sperm production, sperm transfer, mating behaviour and growth. Both types of gamete production were significantly reduced by higher temperature, leading to an overall reduction of reproductive investment. By quantifying sex allocation, we furthermore revealed that the heat-stressed snails reduced the relative investment in their male function. In addition, even though sperm production and its transfer were drastically decreased by high temperature, male mating motivation was not affected. This study illustrates that examining simultaneous hermaphrodites can provide significant insights for the impact of heat, and the proximate mechanism, on reproduction in wildlife.