The incidence of open marriage is the frequency with which open marriage occurs.

Several definitional issues complicate attempts to determine the incidence of open marriage. People sometimes claim to have open marriages when their spouses would not agree. Couples may agree to allow extramarital sex but never actually engage in extramarital sex. Some researchers define open marriages in narrower terms than others. Despite these difficulties, researchers have estimated that between 1.7 percent and 6 percent of married people are involved in open marriages. The incidence of open marriage has remained relatively stable over the last two generations.