The first legal same-sex marriages in the U.S. are performed in the state of Massachusetts.

Same-sex marriage, also known as gay marriage, is the marriage of two people of the same legal sex. As of 2025, marriage between same-sex couples is legally performed and recognized in 38 countries, with a total population of 1.5 billion people. The most recent jurisdiction to legalize same-sex marriage is Thailand.

Same-sex marriage has been legally recognized in Massachusetts since May 17, 2004, as a result of the Massachusetts Supreme Judicial Court (SJC) ruling in Goodridge v. Department of Public Health that it was unconstitutional under the Constitution of Massachusetts to allow only opposite-sex couples to marry. Massachusetts was the sixth jurisdiction in the world to legalize same-sex marriage after the Netherlands, Belgium, Ontario, British Columbia, and Quebec. It was the first U.S. state to open marriage to same-sex couples.