The innate immune system senses pathogens through pattern-recognition receptors (PRRs) that signal to induce effector cytokines, such as type I interferons (IFNs). We characterized IFN-epsilon as a type I IFN because it signaled via the Ifnar1 and Ifnar2 receptors to induce IFN-regulated genes. In contrast to other type I IFNs, IFN-epsilon was not induced by known PRR pathways; instead, IFN-epsilon was constitutively expressed by epithelial cells of the female reproductive tract (FRT) and was hormonally regulated. Ifn-epsilon-deficient mice had increased susceptibility to infection of the FRT by the common sexually transmitted infections (STIs) herpes simplex virus 2 and Chlamydia muridarum. Thus, IFN-epsilon is a potent antipathogen and immunoregulatory cytokine that may be important in combating STIs that represent a major global health and socioeconomic burden.