Can a prenup be thrown out in a divorce?
Yes, but successful challenges are rarer than the search results imply, and they almost never turn on the agreement being unfair. Courts enforce valid prenups routinely. What gets an agreement thrown out is a defect in how it was signed - and in many states, an agreement that was signed properly stands even if its terms now look lopsided.
That single fact should organize how you think about your own, whichever side of it you are on.
What actually voids a prenup
The recurring grounds, everywhere:
It was not signed voluntarily. Duress, undue influence, no real opportunity to review. The classic pattern is the agreement produced days before the wedding with guests already flying in - which is why some states now build in a cooling-off period. California requires at least seven days between the final agreement being presented and signed, and requires independent counsel or an express written waiver of it.
There was no honest financial disclosure. A prenup allocates rights people cannot value without knowing what exists. Hiding assets or income at signing is the most reliable way to lose the agreement later - unless disclosure was knowingly waived in writing, which some states allow.
It is unconscionable - but note the fine print. In states that follow the older uniform act, unfairness alone is not enough: the agreement must have been unconscionable and signed without disclosure or knowledge, so an eyes-open bad deal, properly signed, generally holds. States on the newer 2012 version are much friendlier to a challenger. There, thin financial disclosure can void the agreement by itself, so can having had no real access to a lawyer of your own, and a term that was unconscionable when you signed it can be refused on that alone. Which version your state adopted is the first thing to find out, because it decides whether you have to prove one thing or two.
A required formality is missing. Not in writing, not signed, or - where the state requires it for certain terms - no independent lawyer.
What does not void a prenup: regret, the passage of time, or the marriage turning out badly. And there is no automatic expiration - "do prenups work after ten years" has a one-word answer, yes, unless the agreement itself contains a sunset clause.
The clauses that were never enforceable anyway
Child support and custody cannot be pre-contracted. The child's right to support cannot be adversely affected by the parents' agreement, and custody is decided on best interests at the time - everywhere. Any prenup term to the contrary is ignored, though the rest of the agreement usually survives.
Infidelity penalty clauses fail in California. California's appellate courts struck a $50,000 cheating penalty as contrary to the public policy of no-fault divorce (Diosdado, which was a clause in a mid-marriage agreement, though the same reasoning has since been applied to prenups). Other states have not all landed the same way, so do not assume a clause like this is dead, or alive, where you live. Related and important: cheating does not void a prenup either. The grounds above are about the signing, not about who behaved badly during the marriage. (Whether an affair matters to support outside the prenup is a different question with a state-specific answer.)
Alimony waivers exist under special guardrails. Most states allow them, but this is where courts look hardest. California makes a support waiver unenforceable if the waiving spouse had no independent lawyer at signing - and separately if the waiver is unconscionable at the time of enforcement, a test no amount of lawyering at signing can cure. That is the answer to "can a prenup leave a spouse with nothing": courts keep a backstop, and a waiver that would leave someone destitute after a long marriage is exactly what the backstop is for.
The death trap almost nobody knows
Prenups routinely waive inheritance rights - the surviving spouse's elective share - and those waivers are generally enforceable. But there is a federal exception with teeth: a prenup cannot waive survivor rights in an employer retirement plan. Federal law requires that waiver to be signed by a spouse, and the person signing a prenup is not a spouse yet. Courts have enforced this literally, paying the surviving spouse despite a clear prenup saying otherwise (Hurwitz v. Sher).
The fix is procedural and easy to miss: after the wedding, the waiving spouse signs the plan's own spousal-consent form, in writing, witnessed by a notary or someone from the plan. Ask the plan what kind of plan it is while you are there. On a pension, or any plan that can pay out as a lifetime annuity, a consent signed before the year the employee spouse turns 35 quietly stops counting on the first day of that year and has to be signed again. Nobody sends a reminder, and most 401(k) plans are exempt from that rule, which is why it is a question for the plan administrator rather than a guess. If your prenup dealt with retirement accounts and nobody did that step, the beneficiary rules are worth reading today.
If you are the one challenging
Be honest with yourself about which ground you actually have. "It's unfair" is usually not one; "I was handed it at the rehearsal dinner with no lawyer and no disclosure" is. The evidence lives in the signing history: drafts, emails, the disclosure schedules, who had counsel and when. Timing matters too - raise the challenge in the divorce, not after judgment.
If you are the one defending it
The same history is your case. Keep everything from the signing period, and understand that the challenge will target process, so the record of disclosure, counsel, and time-to-review is what wins. If the agreement was done carefully, the odds favor you heavily.
If there is no prenup
Then your state's default rules decide, which is most people's situation: what gets divided and how, and what stays yours.
Enforceability standards vary by state - roughly half follow a uniform act pattern and the rest apply their own case law, with the same themes (voluntariness, disclosure, conscionability) recurring everywhere. The specifics cited here are California's, labeled as such. Whether your agreement holds is a question for a licensed family-law attorney in your state, with the agreement and its signing history in hand.