A marital property partition agreement—also known as a partition and exchange agreement, a postmarital agreement, or a postnuptial agreement—is an agreement between spouses during marriage to convert marital property (also known as community property in some states) to one spouse’s separate property.

Property that is jointly owned by spouses or domestic partners (often owned as joint tenants with a right of survivorship) may also be partitioned or divided in a lawsuit or court action rather than by agreement of the spouses.

Laws regarding marital property partition agreements vary from state to state and are generally located in a state’s statutes—often in the family code or domestic relations code.

In New York, marital property is not recognized as community property, as New York is an 'equitable distribution' state rather than a community property state. This means that during a divorce, marital property is divided in a way that is equitable, which may not necessarily be equal. A marital property partition agreement, known in New York as a postnuptial agreement, is a legal document where married spouses agree on how to divide their property in the event of a divorce or death. Such agreements can include the division of both current and future assets and must be in writing, signed by both parties, and acknowledged in the manner required to entitle a deed to be recorded. These agreements are subject to certain legal requirements to ensure fairness and voluntariness, and they can be challenged in court if one party claims the agreement was the result of duress, coercion, or fraud. If spouses cannot agree on the division of property, a New York court will divide the property equitably, considering factors such as the length of the marriage, the income and property of each party, and the needs of any children from the marriage. It is advisable for individuals considering a postnuptial agreement to consult with an attorney to ensure that their rights and interests are adequately protected.