A postnuptial agreement does what a prenup does — sets financial and property terms between spouses — but it's signed after the wedding, which means it can respond to things a prenup couldn't have anticipated.
Common reasons couples sign one
A significant change in finances, one spouse starting or selling a business, an inheritance, or simply deciding as a couple that they want clearer financial terms are all common reasons to formalize a postnup mid-marriage.
Rebuilding after a rough patch
Postnups are sometimes used to rebuild trust after infidelity or financial disagreements, setting out consequences or protections both spouses agree to going forward.
Enforceability gets scrutinized more closely
Courts sometimes look harder at postnups than prenups, since a marriage already exists and one spouse may feel more pressure to sign. Full financial disclosure and fair terms for both sides matter even more here.
What it can't override
Like a prenup, a postnup generally can't predetermine child custody or support — those stay tied to the child's best interests at the time they're actually decided.