This systematic review examines the relationship between religion and sexual HIV risk behavior. It focuses primarily on how studies have conceptualized and defined religion, methodologies, and sexual risk outcomes. We also describe regions where studies were conducted and mechanisms by which religion may be associated with sexual risk. We included 137 studies in this review, classifying them as measuring: (1) only religious affiliation (n = 57), (2) only religiosity (n = 48), and (3) both religious affiliation and religiosity (n = 32). A number of studies identified lower levels of sexual HIV risk among Muslims, although many of these examined HIV prevalence rather than specific behavioral risk outcomes. Most studies identified increased religiosity to be associated with lower levels of sexual HIV risk. This finding persists but is weaker when the outcome considered is condom use. The paper reviews ways in which religion may contribute to increase and reduction in sexual HIV risk, gaps in research, and implications for future research on religion and HIV.