Further to the recent U.S. Supreme Court cases recognizing same-sex marriages, the IRS has issued final regulations that codify same-sex marriages for federal tax purposes, regardless of gender. Under Treasury Regulations § 301.7701-18(a), the terms "spouse," "husband," and "wife" mean an individual lawfully married to another individual, and the term "husband and wife" means two individuals lawfully married to each other.
Different jurisdictions have different rules and definition for marriage - what applies for federal tax purposes? The regulations adopt two different rules to determine when there is a "marriage." For persons married in the U.S., there will be a marriage if it is recognized by the state, possession, or territory of the U.S. in which the marriage takes place. The domicile or residence of the parties is not relevant - the U.S. will focus on the place of celebration. For foreign marriages, it will be recognized if the relationship is denominated as marriage and would be recognized as marriage under the laws of at least one state, possession, or territory of the U.S.
Note that legal relationships that are not denominated as marriage, such a registered domestic partnership, civil union, or other similar relationship, will NOT be treated as a marriage.
T.D. 9785, 08/31/2016, Reg. § 1.7701-1, Reg. § 20.7701-2, Reg. § 25.7701-2, Reg. § 26.7701-2, Reg. § 31.7701-2 , Reg. § 301.7701-18