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 Maine, which is not a community property state but rather an 'equitable distribution' state, marital property is not automatically considered to be owned jointly by both spouses. Instead, during a divorce, marital property is divided in a manner that the court deems fair, which may not necessarily be equal. A marital property partition agreement in Maine, also known as a postnuptial agreement, allows married spouses to agree on how their property, which would otherwise be subject to division upon divorce, should be classified and divided. This agreement can convert marital property into the separate property of one spouse. Such agreements must be in writing and are subject to certain legal requirements to be enforceable, including full disclosure of assets and voluntary signing without coercion. If spouses wish to partition or divide jointly owned property without an agreement, they would typically need to seek a court order to do so. The relevant laws can be found in Maine's statutes under the Maine Revised Statutes, particularly in Title 19-A, which pertains to domestic relations.