An open marriage is a marriage where both parties agree that they are permitted the right to have sexual relationships outside the marriage, without regarding this as sexual infidelity. Open marriage can be regarded as a half-way state between traditional marriage and free love. However, the idiosyncrasies of each open marriage are defined by the individuals involved. There is no one set of rules or boundaries for an open marriage; each marriage is unique in defining what works for that couple, at that given time. Also, the rules are subject to change over time as each person and their relationships grow. This change is necessary to adapt to new circumstances that arise. Many therapists familiar with this kind of relationship advocate the idea that the key to any relationship, especially an open marriage, is honest and open communication between all parties involved.

The practice of extra-marital sex is often illegal in jurisdictions where adultery is illegal, regardless of whether the partner(s) have given their consent. Open marriage is not the same thing as polygamy, where sexual relationships are kept entirely within the parties to the marriage.

The concept can be applied to non-marriage relationships; in such circumstances it is known as an open relationship.

Famous people who had open marriages include Dmitri Shostakovich and his wife, and Edna St. Vincent Millay and her husband.