Do I file jointly if I’ve lived with my partner the entire year but we’re not married?

[Butty-012249]

Depends on your State of residency. Some states do recognize common-law marriages. Check your state's local laws. 

[ErnieS0]

It depends on what you mean by “lived with my partner."

If you are just “living together” without any type of formally recognized arrangement then you can never file together.

For federal tax, you can’t file a joint return unless you are married. The IRS does not recognize registered domestic partnerships, civil unions, or other similar formal relationships that are not marriages under state law.

Some states do recognize common law marriages (as awesome community member @Butty-012249 says), along with other arrangements.

Learn more at: 

Answers to Frequently Asked Questions for Registered Domestic Partners and Individuals in Civil Unio...

State Tax Filing for Same-Sex Couples

[Hal_Al]

Q. Do I file jointly if I’ve lived with my partner the entire year but we’re not married?

A. No.  You must be legally married. 

You may be able to file as Married Filing Jointly if you live in a state that recognizes common law marriage. Currently, the following jurisdictions recognize common law marriage:

 Colorado

 District of Columbia

 Iowa

 Kansas

 Montana

 New Hampshire

 Oklahoma

 Rhode Island

 South Carolina

 Texas

 Utah

Alabama (2017), Georgia(1997), Idaho(1996), Ohio(Oct. 1991), and Pennsylvania(2005) are grandfathered for the marriages before the year indicated. Living together in a common law state is usually insufficient - you need to hold yourself out as married including owning property together, having joint bank accounts, etc. To find out your state’s rules see:

http://family.findlaw.com/marriage/common-law-marriage-states.html