Is It Legal to Marry Your Third Cousin? Law for 2026

Marrying your third cousin is legal in all 50 US states. No state consanguinity law prohibits third cousin marriages, and no state has ever enacted one. This is one of the clearest questions in American family law: third cousins are simply not covered by any marriage restriction anywhere in the country.

What confuses people is that cousin marriage laws do exist, and they are significant. The laws focus on first cousins and sometimes second cousins. Third cousins sit so far outside the prohibited zone that most state statutes don’t even mention them.

Most people asking this question don’t realize how distant a third cousin actually is genetically and legally. Third cousins share great-great-grandparents. That is four generations of separation.

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This article covers exactly why third cousin marriage is universally legal, what consanguinity laws actually prohibit, how first and second cousin rules compare, and what the practical legal picture looks like for anyone considering this type of marriage.

Is It Legal to Marry Your Third Cousin?

Marrying your third cousin is fully legal in every US state. No consanguinity statute in any US jurisdiction prohibits marriage between third cousins, and no such prohibition has ever existed in American law at the state or federal level.

Third cousins are legally so distant from each other that they fall entirely outside the prohibited degrees of kinship defined in state marriage statutes. Every state that restricts cousin marriage draws its line at first cousins, and many of those restrictions include exceptions for older couples or infertile couples. Second and third cousins are not reached by these statutes at all.

The question of third cousin marriage often arises because people are uncertain where the law draws the line. The answer is clear and consistent. The legal concern in consanguinity law is about close relatives sharing genetic material at a meaningful level. By the third cousin level, that overlap is genetically minimal and legally irrelevant.

Quick Facts:

• Third cousin marriage is legal in all 50 US states.
• No state consanguinity statute mentions or prohibits third cousin marriage.
• The most restrictive US states prohibit only first cousin marriages.
• Third cousins share approximately 0.78% of their DNA on average.
• A marriage license between third cousins will be issued in any US state.
• No criminal penalties exist for third cousin marriage anywhere in the US.

Third Cousin Marriage Laws by State

Third cousin marriage laws by state are effectively nonexistent because no US state has enacted a statute addressing this degree of kinship. The state-by-state variation in cousin marriage law applies exclusively to first cousins and, in rare cases, to second cousins or half first cousins.

The legislative history of consanguinity restrictions in the US shows that lawmakers drew lines based on genetic proximity and social concerns about close family relationships. Third cousins share only great-great-grandparents. That level of genealogical distance has never prompted legislative concern in any US state, and no advocacy group has ever successfully pushed for a third cousin marriage restriction.

Because no state law addresses third cousin marriage specifically, the marriage licensing process for third cousins is identical to the process for any two unrelated people. Clerks do not ask about kinship beyond the prohibited degree in most states, and no state requires couples to prove they are not third cousins before issuing a license.

Every column in the third cousin category is identical: legal. The variation between states applies only to first cousins and the conditions under which those marriages may or may not be permitted.

How Closely Related Are Third Cousins Legally?

Third cousins are legally defined as the children of second cousins, meaning they share a common set of great-great-grandparents. In legal terms, this places third cousins outside every consanguinity statute currently in force in any US jurisdiction.

Consanguinity in law refers to the degree of biological relationship between two people. US states measure this using a degree system (counting each generational step up and down) or a generation-based system (counting the number of generations to a common ancestor). Under either system, third cousins fall at a degree of relationship so distant that no state has ever defined it as prohibited.

Think of it like zoning laws. Prohibitions on building in certain areas exist for specific parcels. Third cousin marriage sits so far outside the restricted zone that it doesn’t even appear on the regulatory map.

Cousin relationship definitions:

At 0.78% shared DNA, third cousins are genetically closer to random strangers than to close relatives in terms of genetic health implications. The legal system reflects this reality by treating third cousins as legally unrelated for marriage purposes.

Key Takeaway: Third cousins share great-great-grandparents and approximately 0.78% of their DNA. No US state consanguinity law reaches this degree of kinship, making third cousin marriage universally legal.

Consanguinity Laws and Prohibited Degrees of Kinship

Consanguinity laws are state statutes that define which blood relatives are too closely related to legally marry. Every US state has some form of consanguinity statute, and every one of them stops well short of reaching third cousins.

The typical consanguinity statute prohibits marriage between ancestors and descendants (parent-child, grandparent-grandchild), siblings (full and half), uncles/aunts and nieces/nephews, and in many states, first cousins. This is the standard prohibited category that appears in virtually every state’s marriage statute.

Some states extend their consanguinity statutes to cover double first cousins (who share both sets of grandparents) or first cousins once removed. No state extends its prohibition to second cousins, and no state has ever prohibited third cousin marriage. The statutes reflect a consensus about where genetic proximity creates meaningful risk and where social concerns about family relationships justify restriction.

Sample state consanguinity statute provisions:

The pattern is consistent. The most restrictive states draw the line at first cousins. Third cousins appear nowhere in any of these statutes.

Is Third Cousin Marriage Legal in All 50 States?

Third cousin marriage is legal in all 50 US states without exception, qualification, or special conditions. No state requires a court order, a physician’s certificate, or any special permission for third cousins to marry.

This is one of the most consistent rules in all of American family law. While first cousin marriage laws vary widely from state to state, with some states permitting it fully, some prohibiting it entirely, and some allowing it with conditions, third cousin marriage produces the same answer everywhere.

The uniformity makes practical sense. A third cousin is so genealogically distant that the relationships between them and your immediate family are effectively the same as those between random members of the general public in terms of genetic and social proximity. The law treats them accordingly.

Third cousin marriage legal status across all state categories:

• No state has a consanguinity prohibition covering third cousins.
• No state requires genetic counseling before a third cousin marriage.
• No state requires court approval for a third cousin marriage.
• No state treats a third cousin marriage as void or voidable on consanguinity grounds.
• Marriage licenses are issued to third cousins through the same standard process as any other couple.

The practical experience of applying for a marriage license as third cousins will be identical to any other couple’s experience. No clerk will ask whether you are related beyond the prohibited degree, and no form requires you to certify your genealogical relationship beyond the most immediate prohibited categories.

First Cousin Marriage Laws by State

First cousin marriage is where US state law gets genuinely complicated, and understanding this contrast helps explain why third cousin marriage is universally unconcerning to lawmakers. First cousins share approximately 12.5% of their DNA, which is the threshold where state legislatures have historically intervened.

Twenty-five states prohibit first cousin marriages. The remaining 25 states, plus Washington DC, permit them either fully or with conditions. The conditions in some states include requirements that the couple be past childbearing age or that one partner be infertile, requirements reflecting concern about genetic health risks rather than social taboo.

States that prohibit first cousin marriage include Texas, Ohio, West Virginia, Virginia, North Carolina, Michigan, Indiana, Wisconsin, and others. States that permit it include California, New York, Florida, Georgia, and many others. This is where the actual variation in US cousin marriage law sits.

First cousin marriage status across major states:

The range of approaches to first cousin marriage reflects genuine policy disagreement. No such disagreement exists for third cousins because no state has ever argued for their restriction.

Key Takeaway: First cousin marriage is prohibited in 25 states and permitted in 25 others, often with conditions. Second and third cousin marriages are legal everywhere. The legal line in US cousin marriage law sits at first cousins.

Is Second Cousin Marriage Legal in the US?

Second cousin marriage is legal in all 50 US states. Like third cousins, second cousins fall outside the prohibited degrees of kinship defined in every state consanguinity statute. No state has ever enacted a law prohibiting second cousin marriage.

Second cousins share great-grandparents and approximately 3.13% of their DNA. At this level of genetic overlap, no state legislature has determined that a restriction is warranted. The historical and ongoing debate about cousin marriage in the US has focused entirely on first cousins; second cousins have never been a legislative target.

The consistent legality of second cousin marriage across all states means that anyone whose family concerns begin at the second cousin level has no legal issue whatsoever. The question of legality only becomes relevant when the relationship is a first cousin, and even then, a significant number of states permit it.

Cousin marriage legality comparison:

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The pattern is straightforward. The legal restriction applies to first cousins in roughly half the states. From second cousins outward, the law is uniformly permissive.

Can You Marry a Half Cousin Legally?

Marrying a half cousin is legal in the US, but the legal analysis depends on whether the state’s consanguinity laws cover half relatives differently from full relatives. Most state statutes use language that covers both full and half relatives equally when prohibiting certain close-relative marriages.

A half first cousin is the child of a half sibling. Half siblings share only one parent rather than two. Half first cousins therefore share only one grandparent rather than two and share approximately 6.25% of their DNA on average, compared to 12.5% for full first cousins.

In states that prohibit first cousin marriages, the prohibition typically applies to both full and half first cousins because the statutes use language like “children of siblings” or “first cousins” without qualification. Some states have specific language addressing half relatives. The practical result is that in states prohibiting first cousin marriage, half first cousins are also prohibited.

Half cousin marriage legal status:

For half second and half third cousins, the question of legality has the same clear answer as for full second and third cousins: completely legal in all 50 states.

Can You Marry an Adopted Cousin?

Marrying an adopted cousin is legally permissible in most US states because adoption severs the legal tie between an adoptee and their biological family while creating a new legal family relationship. The question turns on whether state consanguinity statutes cover relationships created by adoption.

Most state consanguinity laws apply to blood relatives, not to legally created family relationships through adoption. An adopted person is not biologically related to their adoptive family’s cousins, and most state statutes do not extend consanguinity prohibitions to adoptive family relationships beyond the immediate family unit.

Some states have specifically addressed this question. A handful of states have statutes or court decisions indicating that adopted siblings cannot marry each other even though they lack a blood relationship. For cousins in an adoptive family, the prohibition typically does not extend that far.

Adopted relative marriage considerations:

For adopted third cousins specifically, there is no blood relationship and no legal restriction anywhere in the US. The legal status is the same as for any two unrelated people seeking to marry.

Can You Marry a Step Cousin Legally?

Marrying a step cousin is legal throughout the US because step relationships are not blood relationships and are not covered by consanguinity statutes. Consanguinity laws address blood relatives and sometimes adoptive relatives, but step relationships created through a parent’s remarriage create no blood tie.

A step cousin is the child of a step-sibling, or more commonly, refers to the cousin of a step-parent. There is no blood relationship, no legal adoption relationship, and no consanguinity issue. The person is related to you only by virtue of a marriage that created a family connection, not a biological one.

The only legal concern with step-relative marriages would arise if the step relationship involved a direct guardian-ward dynamic or a position of authority that could raise concerns about undue influence or exploitation. For step cousins who are adults meeting independently of any custodial or authority relationship, no legal barrier exists.

Quick Facts:

• Step cousins have no blood relationship.
• Consanguinity laws do not apply to step relationships.
• Marriage between step cousins is legal in all 50 states.
• No state statute addresses step cousin marriage as a restricted category.
• The marriage license process for step cousins is identical to any other couple.

The clear legal reality is that step relationships, including step cousins, are not reached by any marriage restriction in US law.

Key Takeaway: Second cousins, half cousins, adopted cousins, and step cousins can all legally marry in most US states, with only half first cousins in first-cousin-prohibition states creating any legal concern. Third cousins of any variety are universally unrestricted.

Genetic Risk of Third Cousin Marriage

The genetic risk of marrying a third cousin is minimal and is not a factor in US marriage law at any level. Third cousins share approximately 0.78% of their DNA, which creates a very slight increase in the probability of sharing a recessive gene compared to two completely unrelated people.

The genetic concern behind cousin marriage laws is the risk of both partners carrying the same recessive genetic mutation, which could result in offspring with a genetic disorder. This risk is meaningfully elevated for first cousins (sharing 12.5% of DNA) and was the primary driver behind state-level first cousin marriage restrictions beginning in the 19th century.

For third cousins, the genetic overlap is so small that medical and scientific bodies generally do not classify third cousin marriage as carrying elevated genetic risk. The CDC and major genetic counseling organizations focus their cousin marriage guidance on first cousins. Third cousins are not a category that appears in clinical genetic risk counseling guidelines for consanguineous couples.

DNA sharing and genetic risk by cousin type:

The scientific consensus is that third cousin couples seeking genetic counseling would receive the same baseline information as any couple with no known family health concerns, because the genetic overlap is too small to meaningfully change risk calculations.

Void vs Voidable Marriage for Prohibited Relatives

A void marriage is one that is considered never to have existed under the law. A voidable marriage is one that is legally valid until a court annuls it. Understanding this distinction matters for anyone who enters a marriage that may violate consanguinity laws, though it has no relevance for third cousin marriages.

In states where first cousin marriage is prohibited, a marriage between first cousins is typically void from the beginning. It requires no court action to be invalid; it is simply never recognized as a legal marriage. Some states treat it as voidable, meaning it is recognized until a court formally invalidates it.

The void versus voidable distinction matters for property rights, inheritance, and the legitimacy of children born during the marriage. Children born of a void marriage are typically still considered legitimate under most state laws, but the property and inheritance consequences for the spouses themselves can be significant.

Void vs voidable consanguinity marriages:

For third cousins, this entire framework is irrelevant. A third cousin marriage is not void, not voidable, and not subject to any legal challenge based on consanguinity.

What Happens if You Unknowingly Marry a Close Relative?

Unknowingly marrying a close relative who falls within a state’s prohibited degrees of kinship creates a legally complex situation involving void or voidable marriage doctrine, potential annulment proceedings, and in some states, criminal exposure for incest even when both parties acted in good faith.

The good faith exception is critical here. Most states recognize that accidental entry into a prohibited marriage, such as discovering after the fact that a spouse is a closer relative than known, does not carry the same consequences as knowing entry into a prohibited relationship. Courts generally treat good faith unknowing consanguineous marriages as voidable rather than criminally actionable.

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The discovery typically triggers an annulment proceeding rather than a divorce. An annulment declares the marriage void from the beginning, which has different consequences for property division and spousal support than a divorce. Some states handle this through specific consanguinity annulment provisions; others use their general annulment statutes.

Legal steps when a prohibited marriage is discovered:

• Consult state annulment statutes; consanguinity is typically a ground for annulment.
• File for annulment in family court; the marriage is declared void from inception in most states.
• Property acquired during the marriage is addressed through the annulment proceeding.
• Children born during the marriage retain their legitimate status in most states.
• Good faith is a factor that courts consider in determining consequences.
• Criminal incest prosecution is rare when both parties acted without knowledge of the relationship.

Again, this entire framework does not apply to third cousins. No state defines third cousins as within the prohibited degree, so no annulment ground based on consanguinity would ever be available or applicable.

Annulment for a Marriage Between Close Relatives

Annulment on consanguinity grounds is a specific legal remedy available in states where a marriage violates the prohibited degrees of kinship. The process is distinct from divorce because it treats the marriage as if it never legally existed, rather than dissolving a valid union.

State annulment statutes typically list consanguinity as a specific ground, separate from other annulment grounds like fraud, incapacity, or underage marriage. When a court grants a consanguinity annulment, the legal effect is retroactive: the parties are treated as never having been married under state law.

The practical consequences of a consanguinity annulment include the division of assets acquired during the relationship (typically governed by equitable distribution principles despite the annulment), the resolution of any children’s custody and support (unaffected by the annulment), and the restoration of prior name and marital status to both parties.

Consanguinity annulment process overview:

For third cousin marriages, no consanguinity annulment ground exists. A court presented with a petition to annul a third cousin marriage on consanguinity grounds would dismiss it because third cousins are not within any state’s prohibited degrees.

Famous Historical Examples of Third Cousin Marriage

Third cousin marriages have been common throughout history, including among prominent figures whose family relationships are well-documented. These historical examples reinforce the long-standing social and legal acceptance of this level of kinship in marriage.

Charles Darwin married his first cousin Emma Wedgwood in 1839, not his third cousin. That marriage was considered legal and socially acceptable in England at the time. More relevant to the third cousin question: President John Adams and his wife Abigail were third cousins. They shared a common great-great-grandfather in John Alden, the Mayflower passenger.

Franklin D. Roosevelt married Eleanor Roosevelt, who was his fifth cousin once removed, even more distant than a third cousin. That marriage, while notable for the family connection, raised no legal concerns because the relationship was so genealogically distant.

Notable historical cousin marriages:

These examples show that cousin marriages across various degrees of kinship have been historically common and legally uncontroversial at the third cousin level and beyond.

Religious and Cultural Views on Cousin Marriage Law

Religious and cultural attitudes toward cousin marriage vary widely and have directly influenced state consanguinity laws in the US. Understanding this context helps explain why states have such different approaches to first cousin marriage while third cousin marriage remains universally unaddressed.

Many Protestant denominations in the 19th century advocated for cousin marriage restrictions as part of broader social reform movements. These religious influences helped drive the first cousin marriage prohibitions enacted by many northern and midwestern states between 1850 and 1930. Southern states, which maintained different social norms and were less influenced by these reform movements, mostly retained permissive cousin marriage laws.

Islamic law permits first cousin marriage and even encourages it in some cultural traditions. Jewish law prohibits certain close relative marriages but is generally permissive toward cousin marriage. Catholic canon law has historically required dispensation for first cousin marriages but allows them. These diverse religious frameworks have contributed to the varied state law landscape.

Religious frameworks on cousin marriage:

No major religious tradition prohibits third cousin marriage. The cultural and religious variation that creates legal differences at the first cousin level simply does not exist at the third cousin level.

Frequently Asked Questions

Is it legal to marry your third cousin in the US?

Yes, marrying your third cousin is completely legal in all 50 US states.
No state consanguinity statute reaches the degree of kinship represented by third cousins.
Marriage license clerks will process a third cousin couple’s application through the same standard process as any other couple.
There are no conditions, exceptions, or special requirements for third cousin marriages in any US jurisdiction.

Do any US states prohibit third cousin marriage?

No US state prohibits third cousin marriage.
State consanguinity laws draw their prohibited degree lines at first cousins in the most restrictive states; no state has extended those restrictions to second or third cousins.
The legal prohibition on certain cousin marriages in the US applies only to first cousins in approximately 25 states.
Third cousins are legally treated as unrelated individuals for marriage purposes in every US jurisdiction.

What is the genetic risk of marrying a third cousin?

The genetic risk of marrying a third cousin is negligible under current medical and scientific standards.
Third cousins share approximately 0.78% of their DNA, which creates an extremely small increase in the probability of shared recessive genetic mutations compared to two unrelated people.
Major medical and genetic counseling organizations do not classify third cousin marriages as carrying clinically elevated genetic risk.
Genetic counseling guidelines that address consanguineous couples focus on first cousins; third cousins are outside the clinical concern threshold.

What happens if you marry a relative closer than legally allowed?

Marrying a relative within the prohibited degrees of kinship typically results in the marriage being void from the beginning under state law.
Annulment on consanguinity grounds is the legal remedy, treating the marriage as if it never legally existed.
In some states, knowing entry into a prohibited consanguineous marriage constitutes incest, a criminal offense, in addition to the marriage being void.
Third cousin marriages are entirely unaffected by this framework because no state prohibits them.

Can you marry an adopted cousin or a step cousin legally?

Adopted cousins and step cousins can legally marry in most US states because consanguinity laws address blood relationships, not relationships created by adoption or step-family connections.
For adopted first cousins in states where first cousin marriage is prohibited, there is some legal uncertainty depending on how the state statute is worded.
Step cousins have no blood relationship and are not covered by any consanguinity statute in any US state.
Third cousins, whether biological, adopted, or step, can legally marry everywhere in the US without restriction.

The legal answer to third cousin marriage is unusually simple for a family law question: it’s legal everywhere, no exceptions, no special conditions. The complexity in cousin marriage law lives entirely at the first cousin level, and even there, half of US states permit it.

Bookmark this article if the question comes up again. State marriage laws do update periodically, but the universal legal permission for third cousin marriage has been consistent throughout US legal history and shows no signs of changing.

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