Are Postnuptial Agreements Enforceable? 7 Mistakes to Avoid

A postnuptial agreement can give you financial clarity in your marriage, but will yours hold up in court? 

If done right, yes. But if you make mistakes, it could be thrown out entirely, leaving you or your spouse financially exposed.

These seven common mistakes often lead to a postnup being invalidated. Avoiding them is the key to ensuring your agreement is legally sound and enforceable.

Are Postnuptial Agreements Enforceable in California?

California courts do enforce postnups, but they don’t treat them like any other contract. Unlike prenups, which are signed before marriage, postnups are created between spouses who already have legal obligations to each other. That means courts examine them more closely to ensure fairness and transparency. 

For a postnup to hold up in California, it must meet strict legal standards. Both spouses must sign voluntarily, with full financial disclosure and fairness at both signing and enforcement. 

Why Courts Scrutinize Postnups More Than Prenups

While both prenups and postnups are carefully reviewed, courts tend to scrutinize postnups even more closely, and for good reason.

• For one, married couples owe each other a fiduciary duty, meaning they have to be completely honest about their finances and act in each other’s best interest. If one spouse hides assets or pushes for an unfair deal, the postnup can easily be thrown out.
• Then there’s the issue of power dynamics. A prenup is signed when both partners still have the option not to marry—that’s a huge bargaining chip. However, with a postnup, the couple is already legally tied together, and one spouse might have more financial control than the other. Courts want to make sure no one is pressured to sign just to keep the peace.
• Timing also matters. If it comes out of nowhere during a marital dispute, or right before a big financial decision like selling a business or receiving an inheritance, it raises red flags. Judges will ask: Was this fair? Or was one spouse taking advantage of the situation?

This is why they aren’t just contracts—they’re legal agreements that courts examine from every angle.

7 Mistakes That Can Invalidate Your Postnup

1.  Failing to Disclose All Assets and Debts

California law requires spouses to act as financial partners, not adversaries. That means they must be fully transparent about their assets, income, and debts when signing a postnup. If one spouse hides bank accounts, business revenue, or property, the agreement could be tossed out.

This rule is rooted in fiduciary duty, a legal responsibility that spouses have to treat each other fairly in financial matters. The California Family Code § 721 states that spouses must fully disclose finances in any contract they sign with each other. If a judge finds that one spouse withheld financial information, the agreement is at risk of being invalidated.

In re Marriage of Burkle, a billionaire’s postnup was challenged because he allegedly hid financial details from his wife. The court ruled that postnups require full transparency, reinforcing the importance of financial honesty.

2.  Signing Under Duress, Coercion, or Last-Minute Pressure

A postnuptial agreement must be voluntary. If a spouse signs under pressure, whether emotional, financial, or legal, a judge can throw it out.

Courts look at when and how the agreement was signed. If one spouse sprang the agreement on the other right before a major financial decision like buying a house or starting a business, the agreement might be considered coerced.

3. Agreeing to Unfair or One-Sided Terms

California courts won’t enforce postnups that are outrageously unfair. If one spouse walks away with everything while the other is left with nothing, the agreement will likely be invalidated.

Fairness isn’t just judged at the time of signing. California law requires courts to reassess fairness at enforcement. That means an agreement that seemed reasonable years ago may be thrown out if it leaves one spouse at a severe disadvantage today.

Let’s say Emily and Jake signed a postnup waiving spousal support. At the time, Emily had a solid career and agreed to the terms. Years later, she developed a serious illness and could no longer work. When Jake filed for divorce, he insisted the waiver should stand.

A judge wouldn’t just look at what was fair back then—they’d consider what’s fair now. If enforcing the waiver would leave Emily in severe financial hardship, the court could rule it unconscionable and refuse to uphold it.

4. Not Having Independent Legal Counsel

Unlike prenups, California law doesn’t require each spouse to have their own lawyer for a postnup to be valid. However, not having separate attorneys can still be a big mistake.

If only one spouse had legal representation, the other might later argue that they didn’t fully understand what they were signing. Courts may take that claim seriously, especially if the agreement heavily favors one party.

This is why having independent legal counsel for both spouses is highly recommended. A lawyer ensures that each party understands their rights, reducing the risk of future disputes or legal challenges. 

5. Failing to Follow Proper Legal Formalities

A postnup is more than just signatures on paper—it must be clear, properly formatted, and well-documented to hold up in court. Even small mistakes in execution can make it unenforceable.

Vague terms create legal battles. Saying assets will be divided “fairly” or one spouse will receive “reasonable support” is too ambiguous. Courts expect specific terms—who gets what, who is responsible for debts, and how property will be handled.

Paper trails matter. If a postnup is challenged, courts will want to see proof that both spouses had time to review it, received copies, and understood what they were signing. If there’s no record of this, even a well-drafted agreement can fall apart.

California has its own legal requirements. Some agreements require specific disclosures and waiting periods, depending on the terms. Missing a state-mandated step—even by accident—can void the entire agreement.

If a postnup includes a spousal support waiver, California courts will examine whether both spouses fully understood the financial implications and agreed voluntarily. 

While California Family Code § 721 does not explicitly mention spousal support waivers, it establishes that spouses owe each other a fiduciary duty, requiring full disclosure and fair dealing in financial matters.

If a waiver was signed under pressure, without full transparency, or becomes unfair over time, a judge may refuse to enforce it.

6. Including Illegal or Unenforceable Clauses

While couples can agree on many financial matters in a postnup, some clauses will never hold up in court because they violate California law or public policy. Courts carefully review postnuptial agreements to ensure they don’t overstep legal boundaries.

1. Dictating Child Custody or Child Support

California law is clear: child custody and support decisions must be made in the child’s best interest, not predetermined by a contract between parents. Judges have the final say on custody and support, regardless of what a postnup states.

Even if both parents agree to a specific custody arrangement or child support amount, a court can override it if circumstances change. For example, if an agreement says one parent will always have full custody, but later that arrangement no longer serves the child’s best interest, the court will ignore the clause.

2. Incentivizing Divorce

A postnup cannot encourage or financially reward divorce. This means an agreement that offers one spouse a large payout for filing for divorce or penalizes a spouse financially for staying in the marriage is likely unenforceable.

Courts view such provisions as against public policy because they undermine the principle that marriage should not be entered or exited based on financial incentives.

3. Requiring a Spouse to Do Something Unrelated to Finances

Postnuptial agreements cannot regulate personal behavior within a marriage. While couples can agree on financial rights and obligations, courts do not enforce agreements that:

• Dictate household chores (e.g., one spouse must cook dinner every night)
• Control personal habits (e.g., weight requirements, social media restrictions)
• Require sexual obligations (e.g., frequency of intimacy)

Courts view marriage as a relationship, not a business contract, meaning spouses cannot bargain over personal duties the way they would over financial matters.

7. Not Updating the Agreement When Circumstances Change

A postnup should be reviewed periodically to ensure it still makes sense. If financial situations, careers, or health conditions change, an outdated agreement might not hold up.

Courts consider whether it remains fair over time. If an agreement that once seemed balanced now severely disadvantages one spouse, it could be challenged.

Final Thoughts: Postnuptial Agreement vs. Divorce – Protecting Your Future

A postnuptial agreement can be legally binding, but only if it’s done right. Avoiding these seven mistakes ensures your agreement holds up in court and protects both spouses from unnecessary legal battles.

Avoiding these seven postnup mistakes ensures your agreement remains enforceable. If you’re exploring prenups as well, check out this blog on the biggest mistakes people make with prenuptial agreements.

California judges take a close look at fairness, transparency, and execution when deciding whether to enforce it. If an agreement is rushed, unfair, or missing key legal elements, it could be invalidated—leaving both spouses in a vulnerable position.

If you’re wondering whether your postnuptial agreement will hold up in court, call (310) 820-3500 or schedule a case evaluation online to ensure your agreement is legally sound and built to last.