A prenuptial agreement, antenuptial agreement, or premarital agreement (commonly referred to as a prenup), is a written contract entered into by a couple prior to marriage or a civil union that enables them to select and control many of the legal rights they acquire upon marrying, and what happens when their marriage eventually ends by death or divorce. Couples enter into a written prenuptial agreement to supersede many of the default marital laws that would otherwise apply in the event of divorce, such as the laws that govern the division of property, retirement benefits, savings, and the right to seek alimony (spousal support) with agreed-upon terms that provide certainty and clarify their marital rights. A premarital agreement may also contain waivers of a surviving spouse's right to claim an elective share of the estate of the deceased spouse.

In some countries, including the United States, Belgium and the Netherlands, the prenuptial agreement not only provides for what happens in the event of a divorce, but also to protect some property during the marriage, for instance in case of a bankruptcy. Many countries, including Canada, France, Italy, and Germany, have matrimonial regimes, in addition to, or some cases, in lieu of prenuptial agreements.

Postnuptial agreements are similar to prenuptial agreements, except that they are entered into after a couple is married. When divorce is imminent, postnuptial agreements are referred to as separation agreements.