Four Reasons Why an Illinois Prenup Might Be Unenforceable

 Posted on February 10, 2026 in Prenuptial & Postnuptial Agreements

A prenuptial agreement (prenup) is a premarital contract regarding a couple’s property and assets, including how they will be handled in the event of a divorce. To be legally enforceable in our state, a prenuptial agreement must conform to the legal requirements contained in the Illinois Uniform Premarital Agreement Act. Even a seemingly small error could render a premarital agreement partially or completely unenforceable.

At Goostree Law Group, we can defend the validity of a prenup in court, or attack an unlawful agreement. Our Kane County, IL family law attorneys draw on decades of legal experience, giving us clarity in all matters of divorce.

Four Reasons Why Prenups End Up Thrown Out in 2026

No Formalized, Written Agreement

In some circumstances, you do not necessarily need a written agreement to have a valid contract. An oral contract is sometimes enforceable in Illinois. However, an informal or oral prenuptial agreement is never valid in Illinois. Under state law, a prenup must be "in writing and signed by both parties." An informal pre-marriage "deal" is simply not enforceable. Only written and formalized prenuptial agreements will be enforced by state courts.

Prenup Contains Invalid Provisions

Not only does a prenuptial agreement have to be formal, but the contents must comply with strict state rules and regulations. There are only certain things that can be addressed within a prenup. If an agreement contains invalid provisions, the whole thing might be thrown out by a court. As a simple example, you cannot negotiate terms for child custody or child visitation within a prenup. In addition, you cannot pre-determine child support obligations.

Agreement Signed Under Coercion

Another common reason why prenups are invalidated in Illinois is because of coercion. Under the state’s Uniform Premarital Agreement Act, each spouse must enter into the agreement voluntarily. If the court determines that undue influence or undue pressure was used to get one spouse to sign the agreement, it might render the entire prenup invalid. Both parties should consult with their own family law attorney before signing a prenup.

Lack of Full and Fair Disclosures Before Signing

Finally, prenups can be invalidated on the grounds that full and fair disclosures were not made prior to the marriage. When negotiating a prenup, each party has a basic legal right to know exactly what they are getting themselves into, unless that right is explicitly waived. If one spouse was hiding significant assets, property, or debts from the other, that could be deemed grounds to render the prenuptial agreement invalid and unenforceable.

How Are Prenups Typically Used in Illinois?

In Illinois, prenups are often used when one spouse has more assets, more debt, or a more complex financial life. That could mean a home owned before marriage, a family business, investments, retirement accounts, or expected inheritance.

A prenup can also address student loans, credit card debt, or support for a spouse who pauses a career to raise children. Many couples use a prenup to protect a business, so a divorce does not disrupt employees, partners, or clients. Others use it to protect family property, like farmland or a cabin that has been in the family for years.

A prenup can also help people in second marriages. Many spouses want to protect children from a prior relationship. They may want certain assets to pass to those children, even if the marriage ends or a spouse dies. A prenup can work alongside an estate plan. It can confirm what each spouse intends to keep separate and what will be shared. It can also reduce conflict between a surviving spouse and adult children later.

A good prenup is not only about "who gets what." It is also about expectations. It can set rules for handling joint accounts, major purchases, and savings goals. It can define how to treat bonuses, commissions, and stock grants. It can even address how to handle a home purchase during the marriage. 

What Is an "Unconscionable" Prenup?

In plain terms, an unconscionable prenup is one that is extremely unfair, or so one-sided that a court may refuse to enforce it. Courts look at the facts closely. They focus on what each person knew, what each person signed, and what the agreement actually does.

Unfairness alone is not always enough. Many prenups favor one spouse. That is common when one person has far more assets. The bigger concern is whether the agreement was made in a way that was not honest or not truly voluntary.

Courts also consider whether the terms leave one spouse in a harsh position. For example, if the agreement strips a spouse of any fair share and that spouse would be left without basic support, the court may take a harder look. Our firm can review the terms of your prenup and give you an honest assessment of whether or not the agreement is unconscionable based on the facts. 

Does a Prenup Have to Be Notarized?

A prenup does not usually need to be notarized to be valid in Illinois. The key requirement is that it is in writing and signed by both parties. That said, notarizing can still be a smart step. It can help prove the signatures are real and that each person signed willingly. It can also reduce later claims that a signature was forged or that someone did not understand what was being signed.

Even if notarization is not required, good signing practices matter. Each person should have enough time to review the agreement. Each person should understand the terms. It also helps when both spouses have separate lawyers. That is not a strict requirement in every case, but it can make the agreement much harder to attack later. If you are nervous about signing a prenup, our attorneys can go over the document with you to make sure it represents your interests.

Contact Our St. Charles, IL Family Lawyers

At Goostree Law Group, we have the knowledge and skills to negotiate, draft, review, and litigate prenuptial agreements. If you have questions about prenups or postnups, we are here to help. Contact our firm at 630-584-4800 for a free consultation with an experienced Kane County, IL divorce attorney.