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 Oregon, which is not a community property state but rather an equitable distribution state, marital property is not automatically considered to be owned jointly by spouses. Instead, during 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 Oregon as a postnuptial agreement, is a legal tool that allows married spouses to alter the default rules of property division by mutually agreeing to designate certain marital assets as the separate property of one spouse. These agreements must be in writing and are subject to certain legal requirements to be enforceable, such as full disclosure of assets and voluntary signing without coercion. If spouses cannot agree on the division of property, the courts will decide on an equitable division based on various factors, including the length of the marriage, the contribution of each spouse to the marital estate, and the economic circumstances of each spouse. It's important to consult with an attorney to ensure that a postnuptial agreement is properly drafted and meets all legal requirements.