Common law marriage—also known as marriage without formalities or informal marriage—is marriage without a wedding or marriage ceremony and without a marriage license. Some states recognize common law marriage under certain circumstances—such as when the parties agree to be married, live together (cohabit) after agreeing to be married, and represent to others that they are married (holding themselves out as a married couple).

Other states previously recognized common law marriages, but no longer recognize such marriages after a certain date on which the law was changed—usually by statute. In other states, common law marriage has not been recognized by the legislature in a statute, but the courts have recognized common law marriage in court opinions or case law.

In the state of Washington, common law marriage is not recognized. Washington state law requires that all marriages be solemnized through a formal ceremony and that a valid marriage license be obtained. The state does not provide legal recognition to relationships that would be considered common law marriages in other states, regardless of the length of cohabitation or the couple's representation of themselves as married. However, Washington does recognize common law marriages that were established in other states where such marriages are legally recognized, provided they meet the legal requirements of those states. This recognition is due to the Full Faith and Credit Clause of the U.S. Constitution, which requires states to respect the 'public acts, records, and judicial proceedings' of other states.