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 Hampshire, marital property is not automatically considered community property as New Hampshire is an 'equitable distribution' state rather than a community property state. This means that in the event of a divorce, marital property is divided in a manner that is equitable, but not necessarily equal. A marital property partition agreement, known in New Hampshire as a postnuptial agreement, allows spouses to agree on how their property should be classified and divided. Such agreements can be used to convert marital property into the separate property of one spouse. For a postnuptial agreement to be valid in New Hampshire, it must be in writing and signed by both parties. The agreement must be entered into voluntarily and with full disclosure of each party's financial situation. It is also subject to review by a court to ensure fairness and compliance with the law. If spouses cannot agree on the division of property, a court may partition the property in a lawsuit, taking into account factors such as the duration of the marriage, the age and health of the parties, and their contributions to the marital estate.