All multicellular organisms host microbial communities in and on their bodies, and these microbiomes can have major influences on host biology. Most research has focussed on the oral, skin, and gut microbiomes, whereas relatively little is known about the reproductive microbiome. Here, we review empirical evidence to show that reproductive microbiomes can have significant effects on the reproductive function and performance of males and females. We then discuss the likely repercussions of these effects for evolutionary processes related to sexual selection and sexual conflict, as well as mating systems and reproductive isolation. We argue that knowledge of the reproductive microbiome is fundamental to our understanding of the evolutionary ecology of reproductive strategies and sexual dynamics of host organisms.