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 Washington State, which is a community property state, a marital property partition agreement, also known as a postnuptial agreement, allows married spouses to alter the legal status of their property. Under such an agreement, spouses can agree to convert community property into separate property of one spouse. These agreements must be in writing and are subject to certain legal requirements to be enforceable, including full disclosure of assets and voluntary signing without duress or undue influence. The agreement must also be fair and not promote divorce. If spouses cannot reach an agreement, they may seek a court action to partition or divide the property. The relevant laws can be found in the Revised Code of Washington (RCW), particularly in the sections pertaining to domestic relations and the disposition of property upon dissolution of marriage.