Open marriage is a form of non-monogamy in which the partners of a dyadic marriage agree that each may engage in extramarital sexual or romantic relationships, without this being regarded by them as infidelity, and consider or establish an open relationship despite the implied monogamy of marriage. There are variant forms of open marriage such as swinging and polyamory, each with the partners having varying levels of input into their spouse's activities.

A general definition of an open marriage is that there is an agreement between the two partners to have some degree of sexual interaction outside the couple. There are variant forms of open marriage, each with the partners having varying levels of input on their spouse's activities.

The term open marriage originated in sociology and anthropology. Through the 1960s, researchers used "closed marriage" to indicate the practices of communities and cultures where individuals were intended to marry based upon social conventions and proscriptions, and open marriage where individuals had the ability to make their own choice of spouse.

Open Marriage: A New Life Style for Couples was a best selling book published by M. Evans & Company in 1972 by Nena O'Neill and George O'Neill. It was on the New York Times Best Seller list for 40 weeks. It has been translated into 14 languages and has sold more than 35 million copies worldwide according to the publisher. The book changed the meaning of the term. The O'Neills describe open marriage as a relationship in which each partner has room for personal growth and can individually develop outside friendships, rather than focus obsessively on their couplehood and their family unit (being "closed"). Most of the book describes approaches to revitalizing marriage in areas of trust, role flexibility, communication, identity, and equality. Chapter 16, entitled "Love Without Jealousy", devoted 20 pages to the proposition that an open marriage might possibly include some forms of sexuality with other partners. Fueled by frequent appearances of the O'Neills on television and in magazine articles, the redefinition entered popular consciousness, and open marriage became a synonym for sexually non-monogamous marriage.

In her 1977 book The Marriage Premise, Nena O'Neill advocated sexual fidelity in a chapter of that name. As she later said, "The whole area of extramarital sex is touchy. I don't think we ever saw it as a concept for the majority, and certainly it has not proved to be."

The meaning of open marriage can vary from study to study depending on how the particular researchers have set their selection criteria.

Individuals might claim to have open marriages when their spouses would not agree. Studies and articles that interview individuals without taking their married status into account may not receive accurate information about the actual "open" status of the marriage. Blumstein and Schwartz asked more than 6,000 couples whether or not they had an understanding allowing sex outside their relationship. Interviewed individually, the partners in some couples gave very different responses to this question; the respective replies from one married couple were

Couples may have an open marriage in principle, but not engage in extramarital sex. Studies that define open marriage by agreement alone will tend to report a higher incidence than studies that define open marriage by agreement and behavior. Spaniel and Cole found that 7 percent of couples would consider participating in an open marriage, but only 1.7 percent of couples reported having open marriages that actually included extramarital sex. Blumstein and Schwartz found that 15 percent of married couples share an agreement that allows extramarital sex, but only about 24 percent of men and 22 percent of women (or 6 percent and 5 percent of the total, respectively) who had such an agreement actually engaged in extramarital sex during the prior year.