The National Organization for Marriage (NOM) is an American non-profit political organization established to work against the legalization of same-sex marriage in the United States. It was formed in 2007 specifically to pass California Proposition 8, a state prohibition of same-sex marriage. The group has opposed civil union legislation and gay adoption, and has fought against allowing transgender individuals to use bathrooms that accord with their gender identity. Brian S. Brown has served as the group's president since 2010.

Proposition 8, known informally as Prop 8, was a California ballot proposition and a state constitutional amendment intended to ban same-sex marriage; it passed in the November 2008 California state elections and was later overturned in court. The proposition was created by opponents of same-sex marriage in advance of the California Supreme Court's May 2008 appeal ruling, In re Marriage Cases, which followed the short-lived 2004 same-sex weddings controversy and found the previous ban on same-sex marriage unconstitutional. Proposition 8 was ultimately ruled unconstitutional by a federal court in 2010, although the court decision did not go into effect until June 26, 2013, following the conclusion of proponents' appeals.