What changes when you are already married.
A postnup does the same job as a prenup, but the law treats it differently — and in three states, a great deal more strictly.
A postnuptial agreement does the same job as a prenup — it tells a future court how to divide things — but it is signed by people who are already married. That one difference changes the legal analysis more than most people expect, and in several states it changes it a great deal.
The short version4 points · tap one to jump to it
- Three real differences, all of them in statute — A prenup is supported by the marriage itself and takes effect when you marry.
- Ohio changed its mind, and that matters for what you may have read
- States with a statute that covers postnups directly — Everywhere else, postnups are governed by judge-made law, which means the rules are less predictable and a lawyer's local knowledge counts for more.
- When a postnup is actually the right instrument — A postnup proposed during a crisis, in exchange for staying, is the hardest kind to defend later.
In this subject 4
If the money is already mixedTracing, transmutation and reimbursement decide most postnup outcomes — and four states have rules that are not what you would expect. Ohio prohibited postnups entirely until March 2023.
Three real differences, all of them in statute
The short version4 points · tap one to jump to it
- Three real differences, all of them in statute — A prenup is supported by the marriage itself and takes effect when you marry.
- Ohio changed its mind, and that matters for what you may have read
- States with a statute that covers postnups directly — Everywhere else, postnups are governed by judge-made law, which means the rules are less predictable and a lawyer's local knowledge counts for more.
- When a postnup is actually the right instrument — A postnup proposed during a crisis, in exchange for staying, is the hardest kind to defend later.
In this subject 4
Tracing, transmutation and reimbursement decide most postnup outcomes — and four states have rules that are not what you would expect. Ohio prohibited postnups entirely until March 2023.
1. There is no wedding to serve as consideration
A prenup is supported by the marriage itself and takes effect when you marry. A postnup has no such event behind it. States that have modernised their law solve this by saying so directly — the agreement “is enforceable without consideration” — and by fixing the timing: a premarital agreement is effective on marriage, a marital agreement is effective on signing. In states without such a statute, consideration is a live question.
2. You are no longer strangers, and the law notices
Engaged people are treated as dealing at arm’s length. Married people already owe each other duties of trust and candor, so a postnup gets scrutinized more like a transaction between fiduciaries than a bargain between two independent parties. That is the standard rationale, and it explains why several states impose requirements on postnups that they do not impose on prenups.
An honest note about this one. The fiduciary rationale is stated most famously in appellate case law, and we could not read the leading opinion from an official court source. So rather than cite a case we have not verified, we point instead at the places where legislatures have written the heightened scrutiny directly into statute — below.
3. Some states demand things of a postnup that no prenup requires
Minnesota requires that each spouse be represented by separate legal counsel for a postnup to be valid — not advised to seek it, actually represented — and presumes the agreement unenforceable if either spouse files for divorce, separation or annulment within two years of signing it. Louisiana goes further than any other state: a during-marriage agreement changing the matrimonial regime generally requires a joint petition to the court and a judicial finding that it serves the parties’ best interests. Ohio, which reversed a seventy-year ban in 2023, permits postnups only on conditions written into the statute, including that the terms “do not promote or encourage divorce or profiteering by divorce.”
Ohio changed its mind, and that matters for what you may have read
For seventy years Ohio simply would not enforce postnuptial agreements. Senate Bill 210 changed that effective 23 March 2023, and Ohio spouses may now enter a postnuptial agreement altering their legal relations, and modify or terminate one. A great deal of writing about postnups predates that change and still lists Ohio as a state where they are void. It is out of date.
We could not confirm any state that flatly refuses to enforce postnuptial agreements today. That is a finding worth stating plainly, because “some states don’t allow them” is one of the most repeated claims in this area and we could not verify a current example of it.
States with a statute that covers postnups directly
Everywhere else, postnups are governed by judge-made law, which means the rules are less predictable and a lawyer’s local knowledge counts for more.
When a postnup is actually the right instrument
- One spouse is about to take on significant risk. Starting a business, signing a personal guarantee, taking on a professional liability. A postnup can define what is exposed and what is not, before the risk lands.
- An inheritance or windfall has arrived. Separate property is easy to lose by commingling. An agreement written at the moment it arrives is far cleaner than an argument about tracing a decade later.
- A prenup was never signed and both people wish it had been. This is common and unglamorous. The wedding got in the way, and now there is time.
- The couple is reconciling after a separation. A postnup can be part of a genuine reconstruction — but this is exactly the situation where the two-year presumption in Minnesota exists, and exactly where a court will look hardest at whether one person signed to save the marriage.
- Estate planning for a blended family. Making sure children from a prior marriage receive what was intended, in a way both spouses have agreed to in advance rather than discovered in a will.
The one situation to be careful about. A postnup proposed during a crisis, in exchange for staying, is the hardest kind to defend later. The person who signed will say they signed to save the marriage; that is close to the definition of duress. If you are in that situation, the honest advice is to slow it down, get two lawyers, and put real time between the crisis and the signature — the same things that make a prenup hold, only more so.
The procedural checklist applies here too
Everything that makes a prenup hold up makes a postnup hold up more. Separate counsel, real disclosure, unhurried signing.
The full state-by-state picture
Governing law, waiting periods, counsel requirements and spousal-support rules for 45 of 51 jurisdictions.
Read this part
The Cusp is not a law firm and nothing on this page is legal advice. This is an explanation of how these agreements work and where they fall apart, written so you can have a better conversation with a lawyer — not so you can skip having one. Prenups are one of the few documents where doing it yourself reliably costs more than it saves, because the thing that voids them is almost never the wording. It is the process.
Every rule below is cited to a statute and linked to the state’s own text. Where we could not confirm a state, we say so instead of filling it in.