You can only file taxes jointly if you’re legally married. The IRS restricts Married Filing Jointly to spouses who are married under state law as of December 31 of the tax year, with one narrow exception: couples in a valid common-law marriage. Domestic partners, engaged couples, and long-term partners who share a household file as two separate, unmarried taxpayers, no matter how intertwined their finances are.
The Legal Rule Behind Joint Filing
Federal tax law ties joint returns to marital status, not to how you live or share money. Under 26 U.S.C. § 7703, whether you’re married is determined as of the last day of the tax year. Married on December 31, you qualify for Married Filing Jointly. Not married, you don’t.1Office of the Law Revision Counsel. 26 USC 7703 – Determination of Marital Status
The statute authorizing joint returns, 26 U.S.C. § 6013, requires both spouses to agree to file together. One spouse can have zero income and the couple can still file jointly.2Office of the Law Revision Counsel. 26 USC 6013 – Joint Returns of Income Tax by Husband and Wife
Registered domestic partnerships and civil unions that state law doesn’t classify as marriages don’t count. Neither does cohabitation, no matter how long. If you’re legally separated under a divorce or separate maintenance decree, you’re also not considered married for federal purposes.1Office of the Law Revision Counsel. 26 USC 7703 – Determination of Marital Status
The Common-Law Marriage Exception
There’s one way to file jointly without a marriage certificate. If you entered into a valid common-law marriage in a state that recognizes them, the IRS treats you as married for federal tax purposes and you can file a joint return under § 6013. That remains true even if you later move to a state that requires a formal ceremony.3Internal Revenue Service. Revenue Ruling 2013-17
A common-law marriage generally requires three things: a present agreement between the two of you to be married, cohabitation, and publicly holding yourselves out as a married couple.3Internal Revenue Service. Revenue Ruling 2013-17 Living together for a long time isn’t enough on its own. You need the mutual intent and the public representation.
Roughly a dozen states and the District of Columbia recognize some form of common-law marriage, including Colorado, Iowa, Kansas, Montana, South Carolina, Texas, and Utah. A few others recognize them through case law or for limited purposes only. The question is whether the state where the marriage was established would consider it valid; the IRS defers to that determination.
How Unmarried Partners Actually File
If you don’t have a legal marriage or a valid common-law marriage, you and your partner each file your own return. Your only options are Single or Head of Household, and picking the right one matters.
Single applies if you’re not married, are divorced, or are legally separated by December 31.4Internal Revenue Service. Filing Status Head of Household is the better deal when you qualify. For 2026, the standard deduction for Head of Household is $24,150, which is $8,050 more than the Single deduction of $16,100. The tax brackets are also wider, meaning more of your income is taxed at lower rates.5Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026, Including Amendments From the One, Big, Beautiful Bill
Who Qualifies for Head of Household
Head of Household isn’t just for single parents, but it does require supporting someone. All three of these have to be true:
- You were unmarried on December 31, meaning single, divorced, or legally separated. Certain married individuals who lived apart from their spouse for the last six months of the year and maintained a home for a dependent child can also qualify as “considered unmarried.”
- You paid more than half the cost of maintaining your home, including rent or mortgage, property taxes, insurance, utilities, repairs, and food eaten at home.
- A qualifying person lived with you for more than half the year. The main exception is a dependent parent, who doesn’t have to live with you as long as you pay more than half the cost of maintaining their home.
The qualifying person is typically your child, stepchild, or grandchild who lived with you for more than half the year. A married child counts only if you can claim them as a dependent. Other relatives, such as a sibling or grandparent, can qualify if they lived with you more than half the year and you can claim them as dependents.6Internal Revenue Service. Publication 501 – Dependents, Standard Deduction, and Filing Information
The parent exception is worth knowing. If you pay for a parent’s assisted-living facility or nursing home, that counts as maintaining their home, and you can file as Head of Household even though your parent lives elsewhere.6Internal Revenue Service. Publication 501 – Dependents, Standard Deduction, and Filing Information
Where Unmarried Couples Get Tripped Up
When two unmarried people share a household, tax filing gets messier than most couples expect. Each person files separately and can only claim deductions they’re personally entitled to.
Mortgage interest is the classic problem. To deduct it, you need to be legally liable on the mortgage and have an ownership interest in the home. If only one partner’s name is on the loan, only that partner gets the deduction, even if the other contributes to the monthly payments.7Internal Revenue Service. Publication 936 – Home Mortgage Interest Deduction If both partners are liable, each can deduct the share of interest they actually paid.
Claiming children is another sticking point. When unmarried parents both live with a child, only one can claim that child as a dependent and use them to qualify for Head of Household, the Earned Income Tax Credit, or the Child and Dependent Care Credit. If you can’t agree, the IRS applies tiebreaker rules: the child goes to the parent who lived with the child longer during the year, and if that’s equal, to the parent with the higher adjusted gross income.8Internal Revenue Service. Tie-Breaker Rules
Unmarried couples also miss out on tax breaks built for joint filers. Income phaseout thresholds for many credits and deductions are set higher for joint filers than for single or head-of-household filers, so two people filing separately can hit those cutoffs faster than a married couple with the same combined income.
What Happens if You File Jointly Anyway
Filing a joint return when you’re not legally married isn’t a gray area. If the IRS reclassifies the return, it will recalculate the tax and can assess an accuracy-related penalty of 20% on the resulting underpayment.9Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
The penalty applies when the underpayment stems from negligence or a substantial understatement of tax. For individuals, a substantial understatement means the tax you reported was off by more than 10% of what you actually owed, or by more than $5,000, whichever is larger.9Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments Interest runs on the unpaid balance from the original due date until you settle it.10Internal Revenue Service. Accuracy-Related Penalty
Head of Household draws similar scrutiny, since it’s frequently claimed by people who don’t meet the requirements. If you’re claiming it, keep records showing you paid more than half the household costs and that a qualifying person actually lived with you for the required period.