A postnuptial agreement (sometimes referred to as a post-maritial agreement) is a private contract between a couple who already is married that may define the rights and obligations between the parties concerning how to handle debt, certain assets, future spousal support obligations and property division, as well as certain provisions in the event of a spouse's death.

A prenuptial agreement is an agreement entered into by a couple before the wedding vows and a postnuptial agreement is an agreement entered after the wedding vows. If a marrying couple does not have a legally binding prenuptial or postnuptial agreement, later on, if there is a divorce, the Family Code will control what will happen to parties’ assets and debts. On the other hand, a couple who has a valid prenuptial or a postnuptial agreement can preset the distribution of their assets and debts in the event of divorce. In short, prenuptial and postnuptial agreements permit a couple to "opt out" of the Family Code, and adopt a plan that personally suits their unique objectives and needs.

The most significant difference between prenuptial agreements and postnuptial agreements concerns the fiduciary or confidential relationship the spouses have to one another, created by the marriage under the applicable sections of the Family Code. After marriage both spouses stand as fiduciaries to the other. As a result, each spouse has the duty of highest good faith and fair dealing with the other spouse, and neither spouse can take any unfair advantage of the other.

Included in this fiduciary relationship is the requirement that both spouses provide the other with full disclosure of any and all information pertaining to issues contained in the agreement. Marital agreements, if challenged at a later date, impose on the spouse defending the agreement a greater burden of proof that the agreement is not unfair to the other spouse.

As a result of the legal obligations imposed on married people under the Family Code, postnuptial agreements are more difficult to enforce. The disclosure requirements are more strict, and it's against public policy to encourage divorce. Thus, a party to a postnuptial agreement is likely to challenge it on the grounds that he or she was coerced into signing it upon threat of divorce. While having a postnuptial agreement is almost certainly better than having no agreement at all in the event of divorce, they are much more susceptible to being successfully challenged than a prenuptial agreement. While Family Courts are prone to enforce prenuptial agreements (especially well-drafted balanced agreements in which both parties were represented by counsel), they are inherently skeptical of postnuptial agreements.