The Defense of Marriage Act (DOMA) is a United States federal law passed by the 104th United States Congress and signed into law by President Bill Clinton. It defines marriage for federal purposes as the union of one man and one woman, and allows states to refuse to recognize same-sex marriages granted under the laws of other states. All of the act's provisions, except those relating to its short title, were ruled unconstitutional or left effectively unenforceable by Supreme Court decisions in the cases of United States v. Windsor (2013) and Obergefell v. Hodges (2015), which means the law itself has been practically overturned.

Same-sex marriage emerged as an issue in the late 1980s, drawing opposition especially from socially conservative groups. Congressman Bob Barr and Senator Don Nickles, both members of the Republican Party, introduced the bill that became DOMA in May 1996. It passed both houses of Congress by large, veto-proof majorities, with opposition coming from approximately one-third of the Democratic caucus in both the House of Representatives and the Senate. Clinton criticized the law as "divisive and unnecessary", but nonetheless signed it into law in September 1996.

Section 2 of the act allows states to deny recognition of same-sex marriages conducted by other states. Section 3 codifies non-recognition of same-sex marriages for all federal purposes, including insurance benefits for government employees, social security survivors' benefits, immigration, bankruptcy, and the filing of joint tax returns. It also excludes same-sex spouses from the scope of laws protecting families of federal officers, laws evaluating financial aid eligibility, and federal ethics laws applicable to opposite-sex spouses.: 23–24 After its passage, DOMA was subject to numerous lawsuits and repeal efforts. In United States v. Windsor (2013), the U.S. Supreme Court declared Section 3 of DOMA unconstitutional under the Due Process Clause, thereby requiring the federal government to recognize same-sex marriages conducted by the states. In Obergefell v. Hodges (2015), the Court held that same-sex marriage was a fundamental right protected by both the Due Process Clause and the Equal Protection Clause. The ruling requires all states to perform and recognize the marriages of same-sex couples, leaving Section 2 of DOMA as superseded and unenforceable.