Federal tax law doesn’t itself contain a definition of what constitutes a marriage, rather deferring to state law. As Reg. §301.7701-18(b)(1) provides:
(b) Persons who are lawfully married for federal tax purposes.
(1) In general.
Except as provided in paragraph (b)(2) of this section regarding marriages entered into under the laws of a foreign jurisdiction, a marriage of two individuals is recognized for federal tax purposes if the marriage is recognized by the state, possession, or territory of the United States in which the marriage is entered into, regardless of domicile.
(2) Foreign marriages.
Two individuals who enter into a relationship denominated as marriage under the laws of a foreign jurisdiction are recognized as married for federal tax purposes if the relationship would be recognized as marriage under the laws of at least one state, possession, or territory of the United States, regardless of domicile.
Technical Advice Memorandum 201734007 answers a question regarding whether the IRS had to recognize a “common law” marriage.
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