Most people have heard of a prenup. Far fewer have heard of its lesser-known sibling: the postnup. It covers similar ground, but it's signed at a different point in the relationship, and that difference matters more than it might seem.
This article isn't going to tell you whether a postnuptial agreement makes sense for your marriage or what should go into one. Those are questions for a licensed family law attorney. What it can do is give you a clear, plain English picture of what a postnup actually is, how it differs from a prenup, and why it tends to be held to a stricter standard.
- What a postnuptial agreement actually is
- The real differences between a postnup and a prenup — not just timing
- Why postnups generally face closer legal scrutiny
- When couples typically consider one
- What postnups generally address — and what they generally can't
What a postnuptial agreement is
A postnuptial agreement — informally a postnup — is a written legal contract signed by two people after they're already married. In general terms, it establishes how certain financial matters would be handled if the marriage ends, whether through divorce or death, much like a prenup does.
That's the core of it: a contract, about finances, signed after the wedding rather than before. Postnuptial agreements are recognized in most states, though the specific rules that govern them — what they can cover, what makes them enforceable — vary from state to state, and in a few respects vary from how prenups are treated in the same state.
How a postnup is actually different from a prenup
It's tempting to think of a postnup as simply "a prenup signed late." That's the least useful way to think about it. A few real legal differences generally follow from the timing.
Spouses owe each other a duty prenup signers don't
Before a wedding, two people negotiating a prenup are generally treated by the law as two separate individuals negotiating a contract at arm's length. Once married, spouses generally owe each other a fiduciary duty — an obligation of good faith, fairness, and financial transparency toward one another. Because that duty already exists by the time a postnup is signed, courts in many states apply closer scrutiny to postnups than they do to prenups, precisely because one spouse could otherwise use the trust and power dynamics of an existing marriage to secure an unfair agreement from the other.
Some states require something a prenup doesn't
Contract law generally requires "consideration" — something of value each side exchanges — for an agreement to be enforceable. For a prenup, the marriage itself is typically treated as the consideration each person is exchanging. That reasoning doesn't work for a postnup, because the couple is already married by the time they sign it. In several states, this means a postnuptial agreement generally needs its own independent consideration — something like a mutual release of certain estate claims — to be valid. What counts as adequate consideration for a postnup varies by state and is genuinely unsettled in some of them, which is one more reason this isn't a document to draft without a licensed attorney.
The burden of proof can flip
With a prenup, the spouse who wants to challenge it generally carries the burden of proving it shouldn't be enforced. With a postnup, that burden can shift the other way in some states: the spouse trying to enforce the agreement may need to prove it was entered voluntarily, with full financial disclosure, and on fair terms. That's a meaningfully higher bar, and it's a direct consequence of the fiduciary duty spouses owe each other once married.
| Prenup | Postnup | |
|---|---|---|
| Signed | Before the wedding | After the wedding |
| Legal relationship of the parties | Two individuals negotiating at arm's length | Spouses, who generally owe each other a fiduciary duty |
| Consideration | The upcoming marriage generally serves this role | Often requires separate consideration — varies by state |
| Burden if challenged | Typically on the spouse challenging it | Can shift to the spouse seeking to enforce it |
None of this means postnups are unenforceable or not worth considering. It means they're generally held to a more demanding standard, which makes the process of creating one — full disclosure, independent counsel, no time pressure — even more important than it is for a prenup.
Setup: Priya and Sam have been married four years. Sam starts a business and, two years later, the couple signs a postnuptial agreement stating the business and its future growth belong to Sam individually, regardless of what happens to the marriage.
If the marriage later ends and Priya challenges the agreement, a court reviewing it wouldn't just ask whether Priya signed it. It would generally look at whether Priya had her own independent attorney, whether Sam fully disclosed the business's finances at the time of signing, whether the terms were fair when they were made, and — because postnups often shift this burden — whether Sam (the spouse seeking to enforce it) can affirmatively prove all of that.
A prenup signed under similar circumstances before the wedding would generally face a less demanding version of this same review. That gap is the practical reason a postnup benefits from being treated with even more procedural care than a prenup, not less.
When couples typically consider a postnup
There's no single trigger. A few situations come up often:
- One spouse starts a business after the wedding and wants clarity on how it would be treated
- One spouse receives a significant inheritance or windfall
- One spouse leaves the workforce, often to raise children, and the couple wants to address how that affects their financial picture
- One spouse takes on substantial debt, and the couple wants to be explicit about how it's handled — see how debt is generally treated during a marriage for the background rules a postnup can build on
- A couple is working through a difficult period in the marriage and wants to formalize financial expectations going forward as part of that process
- A couple didn't sign a prenup and, later, wants the kind of clarity a prenup would have provided
Some of these are practical and unemotional. Others come up during genuinely hard conversations. Both are legitimate reasons couples explore a postnuptial agreement, and the document itself doesn't carry a single meaning — its purpose depends entirely on the couple using it.
What postnups generally address — and what they generally can't
Broadly, postnuptial agreements deal with the same categories a prenup does: how property acquired during the marriage would be characterized, how debt would be handled, and in some cases provisions related to financial support if the marriage ends.
What they generally cannot do is the same as a prenup: they cannot predetermine child support or child custody. Courts treat child support as a right belonging to the child, not something parents can bargain away in a private contract, and custody decisions are required to be based on the child's best interests at the time they're made — not locked in years earlier by an agreement.
For background on how property and debt are generally classified during a marriage in the first place, what counts as marital property covers that ground, and applies whether or not a couple ever signs any agreement at all.
This conversation starts with a licensed attorney.
Whether a postnuptial agreement makes sense for your marriage — and what your state requires to make one valid — are questions for a licensed family law attorney in your state.
Understand your state's framework →The conversation is the point, regardless of the document
As with a prenup, the value of considering a postnuptial agreement often extends beyond whatever document results. Working through what each person is bringing to the marriage now, what's changed since the wedding, and what each person considers fair going forward is a genuinely useful conversation — independent of whether it ends in a signed agreement.
Couples who have that conversation openly, with both people's interests genuinely represented, tend to come away with a clearer shared picture of their financial life than they had before. That's true whether or not a postnup is where the conversation ends up. If you want a broader starting point for that kind of conversation, the money conversation most couples skip covers the version of it that doesn't require a lawyer to begin.