An illegal marriage is one that breaks a legal requirement serious enough that the government will not recognize the union: bigamy, marriages between close relatives, underage marriages without the required authorization, marriages entered through fraud, and marriages where one party could not genuinely consent. Depending on the defect, the law either treats the marriage as if it never happened or treats it as valid until a court steps in to end it. The consequences reach well past the relationship itself, from lost spousal rights to criminal prosecution, and they can land on an innocent spouse who had no idea anything was wrong.
Void or Voidable: Why the Difference Controls Everything
Not every illegal marriage works the same way in court. A void marriage has a defect so serious that the law treats it as though the ceremony never happened. Bigamy and incest fall into this category. No lawsuit or court order is technically required for the marriage to be invalid, because it was never valid in the first place. Most people still seek a court declaration anyway, to clean up official records and protect their rights going forward.
A voidable marriage is legally recognized until someone successfully challenges it. Marriages entered under duress, fraud, or while one party lacked mental capacity are usually voidable rather than void. A voidable marriage can also be “ratified” if the injured party keeps living with the other spouse after the problem disappears. Someone coerced into marrying who then stays in the relationship voluntarily for years after the threats stop may be found to have accepted the marriage, which closes off annulment and leaves divorce as the only exit.
Bigamy: Still Married to Someone Else
You cannot legally marry a second person while your first marriage is still in effect. The first marriage has to end through a finalized divorce, a court-ordered annulment, or the death of your spouse. Separation does not count. Living apart for years does not count. Filing for divorce without receiving a final decree does not count. Go through a second ceremony while the first marriage is still on the books, and the second marriage is void from the start.
This rule catches people who assume their situation is “close enough.” Someone who believes an overseas divorce was finalized, or who treats a decades-long separation as an ending, can walk into a bigamous marriage without realizing it. Even a spouse who has been missing for years does not free you to remarry unless you obtain a legal declaration of death or a divorce. The law cares about documentation, not assumptions.
Bigamy is also a crime in every state. Penalties range from a few months to several years in prison, along with fines, depending on jurisdiction. A second spouse who had no knowledge of the first marriage is generally not criminally liable, but the marriage is still invalid.
Marriages Between Close Relatives
Every state prohibits marriages between parents and children, siblings (including half-siblings), and grandparents and grandchildren. These marriages are void regardless of whether either party knew about the relationship. Unlike some other grounds, this one cannot be cured by ratification or the passage of time.
Beyond that universal core, the rules vary. Some states prohibit marriages between aunts or uncles and their nieces or nephews. First-cousin marriage is legal in roughly half the states, restricted in others (for example, allowed only if both parties are above a certain age), and banned outright elsewhere. A few states extend the bar to step-parents and step-children or to adoptive parents and adopted children, treating the legal parent-child tie as enough on its own.
Underage Marriage and Problems With Consent
Eighteen is the standard age for marrying without anyone else’s permission. Below that, most states require parental consent, judicial approval, or both. A minor who marries without the required authorization usually ends up in a voidable marriage: it stands unless challenged. A handful of states have moved to ban marriage under 18 entirely, but most still allow some path for minors with adult or court approval.
Age is only part of consent. Both people have to voluntarily agree to the marriage with a clear understanding of what they are doing. A marriage is illegal on consent grounds in three main situations:
- Duress or coercion, where one party was pressured through threats of violence or other serious harm. Courts require proof that the threats actually caused the marriage, and the standard is higher than for voiding an ordinary contract.
- Incapacity from alcohol or drugs so severe at the time of the ceremony that the person could not understand the commitment.
- Mental incapacity from cognitive disability or mental illness that prevented the person from grasping the nature and consequences of marriage.
Marriages tainted by duress or incapacity are generally voidable. The affected party can seek an annulment, but until that happens, the marriage remains on the books.
Immigration Marriage Fraud
A marriage entered solely to evade immigration law is both void and a federal crime. Congress enacted the Immigration Marriage Fraud Amendments of 1986 to combat fake marriages used to obtain permanent residency.1U.S. Citizenship and Immigration Services. USCIS Policy Manual Volume 6 Part I Chapter 1 – Purpose and Background2Government Publishing Office. Immigration Marriage Fraud Amendments of 1986 If USCIS concludes a marriage was a transaction rather than a relationship, the immigrant spouse’s permanent resident status is terminated and removal proceedings follow.
The criminal exposure is separate. Knowingly entering a marriage to evade immigration laws carries up to five years in prison and fines up to $250,000 under federal law, and the penalty applies to both the immigrant and the U.S. citizen who takes part.3Office of the Law Revision Counsel. 8 USC 1325 – Improper Entry by Alien4U.S. Immigration and Customs Enforcement. Marriage Fraud Brochure If false documents or sworn statements were used along the way, prosecutors can add charges under 18 U.S.C. ยง 1546, which covers immigration document fraud and carries up to ten years for a first or second offense.5Office of the Law Revision Counsel. 18 USC 1546 – Fraud and Misuse of Visas, Permits, and Other Documents A non-citizen convicted of marriage fraud also faces deportation and a permanent bar on future immigration benefits.
If You Married in Good Faith: The Putative Spouse Doctrine
If you married someone in good faith without knowing the marriage was invalid, you may qualify as a “putative spouse.” The classic scenario is marrying someone who turns out to still be legally married to a prior spouse. You had no way of knowing, and the law recognizes that you should not lose everything because of someone else’s deception.
In jurisdictions that recognize the doctrine, the innocent party is entitled to property rights similar to those of a legal spouse, which can include a share of assets acquired during the relationship and spousal support.6Legal Information Institute. Putative Spouse Doctrine Not every state recognizes it, and the specifics vary. The core requirement is good faith: you must have genuinely and reasonably believed the marriage was valid. If you knew or should have known about the defect, you do not qualify.
What Happens to the Children
Children born during a marriage later declared void or annulled do not lose their legal status. Every state has provisions treating these children as legitimate for purposes of inheritance, child support, and parental rights. Both parents remain responsible for support, and both retain parental rights unless a court orders otherwise. Custody and support are handled the same way they would be in a standard divorce, whether the underlying marriage was bigamous, incestuous, or fraudulent.
Tax and Benefit Fallout
When a marriage is declared void, the IRS treats it as though it never existed for tax purposes. Joint returns filed during the marriage become invalid. The IRS allows taxpayers whose marriages are found invalid to amend prior returns even after the normal filing deadline, provided they supply court documentation showing the marriage was not valid.7Internal Revenue Service. 21.6.1 Filing Status and Exemption/Dependent Adjustments Each person has to refile as single (or head of household if they qualify), reallocate income, credits, and payments between the two returns, and submit the amended returns using Form 1040-X.8Internal Revenue Service. File an Amended Return
Social Security creates a separate headache. If you received spousal or survivor benefits based on a marriage that was later invalidated, the Social Security Administration may classify those payments as overpayments and demand repayment. SSA can recover the money by withholding future benefits, intercepting tax refunds, or garnishing wages.9Social Security Administration. Resolve an Overpayment A waiver is possible if the overpayment was not your fault and you cannot afford to repay, but approval is not guaranteed.
Health insurance coverage, pension beneficiary designations, and any other benefit tied to marital status can also unravel. The ripple effects run broader and cost more than most people expect, which is a practical reason to seek a formal court declaration of invalidity even when a marriage is technically already void.
Getting It Made Official: Annulment
Even when a marriage is void by law, most people still need a court order to make it official. An annulment is a court declaration that a valid marriage never existed, which is different from a divorce, which ends a marriage that was valid. For void marriages, the annulment is a formal recognition of what was already true. For voidable marriages, the annulment is what actually invalidates the union.
The process resembles divorce. You file a petition, but instead of citing irreconcilable differences, you have to prove the specific legal defect that made the marriage invalid. That can mean producing a prior marriage certificate, evidence of fraud, medical records showing incapacity, or proof of a prohibited family relationship. The burden of proof sits with the person seeking the annulment, so these cases run more evidence-heavy than a typical no-fault divorce.
Timing matters, especially for voidable marriages. Many states impose strict deadlines based on the ground involved. If you wait too long after discovering the problem, or you keep living as a married couple, you may lose the right to annul and have to file for divorce instead. Filing fees generally track divorce filing fees in the same jurisdiction. The larger expense is usually attorney fees, particularly if the other party contests the annulment or disputes the facts.
Same-Sex Marriage Is Not a Ground
Since the Supreme Court’s 2015 decision in Obergefell v. Hodges, state laws banning same-sex marriage are unconstitutional and unenforceable.10Justia. Obergefell v. Hodges, 576 U.S. 644 (2015) A same-sex marriage that meets every other legal requirement is fully valid. Some states still have pre-Obergefell bans on their books, but those provisions carry no legal weight. A same-sex marriage can still be illegal for the same reasons any other marriage can, including bigamy, incest, lack of consent, fraud, or failure to meet age requirements. Sexual orientation itself is not a legal barrier.