Illinois adultery law technically makes cheating on a spouse a Class A misdemeanor under 720 ILCS 5/11-35, punishable by up to a year in jail and a $2,500 fine, but prosecutions are effectively nonexistent. The law’s practical bite lands elsewhere: in divorce, where affair-related spending can be clawed back through a dissipation claim; in immigration, where an affair that broke up a marriage can block naturalization; and, in narrow circumstances, in parenting decisions when a child was actually harmed.
What the Criminal Statute Says
Three elements have to line up before anyone could be charged under the statute. A person must have sexual intercourse with someone other than their spouse. The behavior must be “open and notorious.” And both people involved must know that at least one of them is married.1Illinois General Assembly. Illinois Compiled Statutes 720 ILCS 5/11-35 – Adultery
The “open and notorious” requirement is the piece most people miss. A private affair that no one in the community knows about would not, on a strict reading, meet this threshold. Illinois courts have never precisely defined how public the conduct needs to be, largely because the law is almost never enforced, but the phrase carries its ordinary legal meaning: visible or known enough that a reasonable person in the community would be aware of it. And if the unmarried partner genuinely didn’t know the other person was married, the knowledge element fails as to that partner.
On paper, a Class A misdemeanor conviction carries up to a year in jail and a fine of up to $2,500.2Illinois General Assembly. Illinois Compiled Statutes 730 ILCS 5/5-4.5-55 – Class A Misdemeanors Sentence A court could also impose probation or conditional discharge for up to two years.3Illinois General Assembly. Illinois Compiled Statutes 730 ILCS 5/5-4.5-55 – Class A Misdemeanors Sentence These numbers exist entirely in theory. There are no reported Illinois appellate decisions sentencing anyone under the statute in recent decades.
Why No One Actually Gets Charged
Prosecutors have broad discretion, and an adultery charge offers no public safety benefit while eating resources. The evidentiary hurdle alone is steep: proving that two adults knowingly had sex, that both understood one was married, and that the behavior was public enough to qualify as open and notorious, all beyond a reasonable doubt, is a case few prosecutors would build.
There is also a constitutional cloud over the law. The U.S. Supreme Court’s 2003 decision in Lawrence v. Texas struck down sodomy laws on privacy grounds, and many legal scholars argue the same reasoning would sink adultery statutes if challenged. No Illinois court has directly ruled on the question, but the possibility gives prosecutors another reason to leave the statute alone.
Where Adultery Actually Matters: Divorce
Illinois is a no-fault divorce state, and the statutes explicitly tell judges to ignore marital misconduct in most of what they decide. That framework does most of the work in answering what adultery “costs” someone in a divorce. There is one real exception, and it involves money spent on the affair.
Dissipation of Marital Assets
Illinois requires courts to divide marital property “without regard to marital misconduct.”4Justia Law. Illinois Code 750 ILCS 5/503 – Disposition of Property and Debts The affair by itself won’t cost a cheating spouse a bigger share of the house or the retirement accounts. But if marital money was spent to fund the affair (hotel rooms, gifts, travel, an apartment), the other spouse can file a dissipation claim asking the court to account for that spending when dividing property.
Dissipation has strict rules. The accusing spouse must file a formal notice at least 60 days before trial, or 30 days after discovery closes, identifying when the marriage began breaking down, what property was dissipated, and when. Claims can’t reach back more than five years before the divorce petition, and no earlier than three years after the accusing spouse knew or should have known about the spending.4Justia Law. Illinois Code 750 ILCS 5/503 – Disposition of Property and Debts Vague allegations don’t survive. You need specific transactions, dates, and dollar amounts, usually pulled from bank records and credit card statements.
Spousal Maintenance
The statute is blunt: maintenance is set “without regard to marital misconduct.”5FindLaw. Illinois Code 750 ILCS 5/504 – Maintenance A cheating spouse doesn’t owe more because of the affair, and the other spouse doesn’t get a larger award as compensation. Income, earning capacity, length of the marriage, and financial need drive the calculation. Adultery is not a factor.
Parenting Time and Decision-Making
Illinois allocates parenting time under a best-interests-of-the-child standard in 750 ILCS 5/602.7. The statute lists 17 factors, and adultery is not among them. It also directs courts not to consider conduct of a parent “that does not affect that parent’s relationship to the child.”6Illinois General Assembly. Illinois Compiled Statutes 750 ILCS 5/602.7 – Best Interests of the Child
An affair can still surface indirectly if the surrounding conduct harmed the children: neglect during the affair, exposure to unsafe situations, a new partner who poses a real concern, or pressuring the children to keep secrets from the other parent. The affair itself isn’t what a court weighs. The impact on the child is.
No Civil Suit Against the Third Party
Some states still let a betrayed spouse sue the person their partner had an affair with under a theory called alienation of affection. Illinois eliminated those claims in 2016. There is no civil cause of action in Illinois against the third party, no matter how egregious the circumstances.
Immigration: Where the Law Has Real Teeth
Naturalization applicants have to demonstrate “good moral character” during the statutory period before applying for citizenship. Federal regulations list an extramarital affair that “tended to destroy an existing marriage” as a conditional bar to meeting that requirement, unless the applicant can establish extenuating circumstances.7eCFR. 8 CFR 316.10 – Good Moral Character
USCIS reviews these cases individually. The question is whether the affair contributed to the marriage’s breakdown, not simply whether the applicant had an outside relationship. A marriage that was already functionally over is the kind of extenuating circumstance that can overcome the bar. An applicant whose affair ended a marriage inside the three-to-five-year statutory window faces a genuine obstacle, and anyone in that position should talk to an immigration attorney before filing.
What a Dissipation Fight Costs
The criminal statute is a paper tiger, but a divorce driven by an affair can become expensive fast. Proving dissipation often requires forensic accountants, subpoenas for financial records, and extended discovery. Some spouses hire private investigators for surveillance, which typically runs $75 to $500 per hour depending on the case.
Divorce filing fees in Illinois generally fall between $250 and $450, but that’s a rounding error once dissipation is in play. Attorney fees for a contested divorce with real financial disputes can reach five figures. The spouse making the dissipation claim carries the initial burden of identifying specific expenditures, which means spending real money on evidence before the court ever weighs it.
The Practical Takeaway
The criminal law against adultery in Illinois is a holdover that no prosecutor is likely to pick up. Its practical consequences are narrow: dissipation of marital assets can shift how property is divided when affair-related spending is documented; naturalization can be blocked when an affair broke up a marriage during the good-moral-character window; and parenting time can be affected only when the surrounding conduct actually harmed the children. Maintenance is off the table, custody-by-punishment is off the table, and a civil suit against the third party is off the table. For most Illinois residents, adultery is a moral and personal issue with a small, specific set of legal edges.