Prenup vs. Postnup in Florida: What’s the Difference?

By Nadia Pazos, Family Law Attorney, Pazos Law Group · Updated July 2026 · ← All blog posts

They sound similar and cover much of the same ground, but a prenuptial and a postnuptial agreement are governed by different bodies of Florida law — and that difference matters if either is ever challenged.

Quick Answer

A prenuptial agreement is signed before marriage and is governed by Fla. Stat. § 61.079. A postnuptial agreement is signed after a couple is already married and is governed by Florida contract law and case law — notably Casto v. Casto, 508 So. 2d 330 (Fla. 1987). Both can define separate versus marital property and set or waive alimony; neither can predetermine child support or custody. In practice, postnups face close scrutiny because spouses already owe each other a duty of good faith.

The Core Difference: Timing and Legal Basis

The obvious difference is when you sign. A prenup is executed before the wedding and only takes effect once you marry. A postnup is executed after you are already married. But the more important difference is the law that governs each.

Prenups are governed by Fla. Stat. § 61.079, Florida’s Uniform Premarital Agreement Act, which sets clear statutory requirements.
Postnups are not covered by § 61.079. They are governed by general contract principles and Florida case law, most importantly Casto v. Casto, 508 So. 2d 330 (Fla. 1987), which sets the framework for setting aside a marital agreement.

Why Postnups Face Closer Scrutiny

Before marriage, two people are dealing at arm’s length. Once married, spouses owe each other a duty of good faith and fair dealing, so Florida courts examine postnuptial agreements carefully. Under Casto, a spouse can challenge a marital agreement by showing it was reached through fraud, deceit, duress, coercion, misrepresentation, or overreaching — or, alternatively, that it is unfair or unreasonable given the parties’ circumstances and was signed without adequate financial disclosure. The practical takeaway: full, honest disclosure and independent counsel matter even more with a postnup.

What Each Agreement Can Do

Both a prenup and a postnup can define what is separate versus marital property, direct how assets pass on divorce or death, protect a business or professional practice, allocate debt, and set, limit, or waive alimony. And both share the same hard limit: neither can predetermine child support or a parenting schedule, which Florida courts decide under the best-interests standard in Fla. Stat. § 61.13.

When Couples Use a Postnup

The wedding arrived before a prenup could be finished, and the couple wants to put the same protections in place now.
One spouse has since started or acquired a business, received an inheritance, or experienced a major change in wealth.
The couple is reconciling after a difficult period and wants to define financial expectations going forward.
Estate planning — a postnup can work alongside wills and trusts to keep certain assets separate for children from a prior relationship.

Which Is Stronger?

All else equal, a properly executed prenup rests on a clear statute and is generally the more predictable instrument. A postnup is fully valid and enforceable in Florida, but because it is judged under Casto rather than a checklist statute, the quality of disclosure and process carries even more weight. If you can address these questions before the wedding, a prenup is usually the cleaner path; if life has already moved on, a carefully drafted postnup accomplishes the same goals.

Frequently Asked Questions

Is a postnuptial agreement enforceable in Florida?

Yes. Florida enforces postnuptial agreements. They are governed by contract law and cases like Casto v. Casto, 508 So. 2d 330 (Fla. 1987), rather than the prenup statute. A postnup can be challenged if it was procured by fraud, duress, coercion, or overreaching, or is unfair and was signed without adequate financial disclosure.

What is the main difference between a prenup and a postnup?

Timing and governing law. A prenup is signed before marriage and governed by Fla. Stat. § 61.079. A postnup is signed during the marriage and governed by Florida contract law and case law (Casto v. Casto). Both can address property and alimony but not child support or custody.

Can we turn our prenup into a postnup, or update it after marrying?

You cannot convert one into the other, but a married couple can sign a new postnuptial agreement that updates or replaces the terms of an earlier prenup. The postnup must meet the standards for marital agreements, including fair disclosure and voluntary execution.

Why would a married couple sign a postnup?

Common reasons include: the wedding arrived before a prenup was finished; one spouse later started a business or received an inheritance; the couple is reconciling and wants clear financial expectations; or the postnup supports an estate plan to preserve assets for children from a prior relationship.

Speak With a Florida Prenuptial Agreement Attorney

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This article is general legal information about Florida law and is not legal advice. Whether a prenuptial or postnuptial agreement is enforceable depends on the specific facts, the parties’ disclosures, and the circumstances of signing. Reading this content does not create an attorney-client relationship with Pazos Law Group. Florida law changes over time; please consult a licensed Florida attorney about your situation.