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 Illinois, marital property is not considered community property, as Illinois is an 'equitable distribution' state rather than a community property state. This means that during a divorce, marital property is divided in a way that is equitable, but not necessarily equal. A marital property partition agreement in Illinois, often referred to as a postnuptial agreement, allows married spouses to agree on how their marital property should be divided in the event of a divorce. This agreement can be used to convert marital property into the separate property of one spouse. For such an agreement to be valid, it must be in writing and signed by both parties. Additionally, the agreement must be entered into voluntarily and without coercion, with full disclosure of assets by both parties. It is important to note that Illinois courts will review postnuptial agreements for fairness at the time of enforcement, and they have the authority to invalidate provisions that are unconscionable or were not entered into with informed consent. 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 length of the marriage, the contribution of each spouse to the marital estate, and the economic circumstances of each spouse.