In order to determine if one partner of an unmarried or cohabiting couple may claim the other partner as a dependent when they file their taxes, the first question to ask is whether they are in fact unmarried.
That is to say, even if the couple has never had a formal wedding ceremony and exchanged vows, they may be in a “common law” marriage. The Internal Revenue Service (IRS) states that for tax purposes you are considered married if you are in a “common law marriage that is recognized in the state where you now live or in the state where the common law marriage began.”
So common law marriage will vary from state to state. Typically it depends on factors such as the length of time you have lived together, and the presence or absence of children. Another factor is whether you are an opposite-sex or same-sex couple, as some states explicitly preclude same-sex couples from the legal definition of marriage.
If by law you are married after all, then you need to consult the tax rules for married couples. There are tax advantages and disadvantages to being married, but one of the implications is that you cannot claim a spouse as a dependent (though you can under certain circumstance claim a personal exemption for a spouse).
But let’s assume that you are indeed unmarried, and return to the original question of whether you may claim your partner as a dependent.
The answer is yes, assuming all of the following conditions are met:
1. Your partner lived with you the entire year.
2. Your partner is a citizen of the United States, Canada, or Mexico, or a resident alien of the United States.
3. You provided at least 50% support for your partner. This would include things like food, shelter, utilities, clothing, schooling, etc. You should have receipts or some kind of evidence to show that you paid for these things, in case of an audit.
4. Your partner’s taxable income for the year was not over the personal exemption amount. This amount can change from year to year. For 2009 and 2010 it was $3,650.
5. Your partner is not married (to someone else) and filing jointly with their spouse, unless that return is filed only to receive a refund.
6. Your partner has not already been claimed as a dependent by someone else for the same tax year.
7. Your relationship with your partner is not in violation of state law. (Some states have laws against cohabitation, sodomy, etc. Whether such laws would actually cause the IRS to disallow your claiming your partner as a dependent nowadays is doubtful.
Incidentally, these are the same rules for claiming an adult dependent in general. So there is nothing special about your being partners; if you were just platonic roommates, adult uncle and nephew, etc., the same conditions would apply for claiming a dependent.
“Can You Claim Your Partner as a Dependent on Taxes?” Tax Attorney.
“Publication 17 (2009).” IRS.gov.