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 Massachusetts, 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, in the event of a divorce, marital property is divided in a manner that is equitable, which may not necessarily be equal. A marital property partition agreement, known in Massachusetts as a postnuptial agreement, allows spouses to agree on how their property, which may currently be considered marital property, can be divided into separate property owned by one spouse individually. These agreements must be fair and reasonable at the time of execution and not the result of fraud, duress, or undue influence. Additionally, both parties must have had the opportunity to consult with an attorney before signing the agreement. If a couple cannot reach an agreement and a lawsuit or court action is necessary, the Massachusetts courts will divide the property in an equitable manner, considering factors such as the length of the marriage, the conduct of the parties during the marriage, and each party's contribution to the marital estate.