What a Postnuptial Agreement Is
A postnuptial agreement, or postnup, is a contract between spouses who are already married, addressing how property, debt, and support would be divided if the marriage later ends in divorce or death. Functionally, it covers much of the same ground as a prenup — the difference is timing, and timing changes the legal analysis considerably.
No Statute, Different Standard
Unlike prenups, which are governed by Florida's Uniform Premarital Agreement Act (Fla. Stat. § 61.079), postnups are not covered by that statute. Instead, Florida courts evaluate postnups under general contract law principles and the framework set out by the Florida Supreme Court in Casto v. Casto, 508 So. 2d 330 (Fla. 1987). Under Casto, a postnup is enforceable where each spouse made a full and fair financial disclosure before signing, the agreement was entered into freely and voluntarily, without fraud, duress, coercion, or overreaching, and the terms weren't unconscionable given the circumstances at the time.
Why Full Financial Disclosure Actually Matters Here
This is the biggest practical difference from a prenup. A prenup can, in some circumstances, be enforced even without full disclosure if the challenging spouse already had a general understanding of the marital finances, or disclosure was validly waived. A postnup gets closer scrutiny: full, fair disclosure of the marital estate, or the challenging spouse's own independent, reasonably detailed knowledge of it, is central to whether the agreement holds up. The disclosure doesn't need to be exact to the dollar, but it needs to give the other spouse a genuine picture of what's being divided or waived.
Why Courts Look Harder at Postnups
Once a couple is married, the law recognizes that spouses already owe each other duties of fairness and good faith that engaged couples don't yet have. That existing relationship creates more room for one spouse to pressure the other, whether through financial leverage, emotional dynamics, or simply the practical difficulty of walking away from an existing marriage. Because of that dynamic, a spouse challenging a postnup can have it set aside by showing it was the product of fraud, duress, coercion, or overreaching, or that it makes an unreasonable provision for that spouse — which shifts the burden to the other spouse to prove adequate disclosure or independent knowledge.
Common Reasons Couples Sign a Postnup
Postnups tend to come up in response to something specific that happens during a marriage rather than as a routine planning step. Frequent scenarios include one spouse starting or selling a business, a significant inheritance or gift one spouse wants to keep separate, a period of marital difficulty — including infidelity — that the couple wants to address with financial terms rather than divorce, one spouse taking on significant new debt, or a couple who simply never got around to a prenup before the wedding and wants similar protections in place now.
What a Postnup Cannot Do
Like a prenup, a postnup cannot predetermine child support or time-sharing arrangements; those remain subject to the court's independent review of the child's best interests at the time they're decided. And a postnup that's fundamentally unfair to one spouse, given the full circumstances at signing, risks being thrown out even with disclosure in place.
Getting It Right the First Time
Because postnups face closer scrutiny than prenups, the details matter even more: complete and documented financial disclosure, each spouse represented by their own attorney, and enough time to review and negotiate the terms without pressure. An agreement drafted carefully at the outset is far more likely to hold up years later if it's ever tested.