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 by the courts and by Proposition 3 which was passed by voters in 2024. 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 found the ban in 2000 on same-sex marriage (Proposition 22) unconstitutional. Proposition 8 was ultimately ruled unconstitutional in 2010 by a federal court on different grounds, although the ruling did not go into effect until June 26, 2013, following the conclusion of appeals.

Proposition 8 countermanded the May 2008 ruling by adding Proposition 22 wording as an amendment to the California Constitution, providing that "only marriage between a man and a woman is valid or recognized in California". It was ruled constitutional by the California Supreme Court in Strauss v. Horton in 2009, on the grounds that it "carved out a limited [or 'narrow'] exception to the state equal protection clause"; in his dissent, Justice Carlos R. Moreno wrote that exceptions to the equal protection clause could not be made by any majority, since its whole purpose was to protect minorities against the will of a majority.

Legal challenges to Prop 8 were presented quickly after its approval. Following affirmation of Prop 8 by the state courts, two same-sex couples filed a lawsuit in the United States District Court for the Northern District of California in the case Perry v. Schwarzenegger (later Hollingsworth v. Perry). In August 2010, Chief Judge Vaughn Walker ruled that Prop 8 was unconstitutional under both the Due Process and Equal Protection Clauses of the U.S. Fourteenth Amendment, since Prop 8 purported to re-remove rights from a disfavored class only, with no rational basis. The official proponents' justifications for Prop 8 were analyzed in over fifty pages covering eighty findings of fact. The state government supported the ruling and refused to defend Prop 8. The ruling was stayed, pending appeal by the proponents of Prop 8. On February 7, 2012, the Ninth Circuit Court of Appeals, in a 2–1 decision, reached the same conclusion as the district court, but on narrower grounds. The court ruled that it was unconstitutional for California to take marriage rights away from same-sex couples shortly after having granted them. The ruling was stayed pending appeal to the United States Supreme Court.

On June 26, 2013, the Supreme Court of the United States issued its decision on the appeal in the case Hollingsworth v. Perry, ruling that proponents of initiatives such as Prop 8 did not possess legal standing in their own right to defend the resulting law in federal court, either to the Supreme Court or (previously) to the Ninth Circuit Court of Appeals. Therefore, the Supreme Court vacated the decision of the Ninth Circuit, and remanded the case for further proceedings. The decision left the district court's 2010 ruling intact. On June 28, 2013, the Ninth Circuit, on remand, dismissed the appeal for lack of jurisdiction and dissolved their previous stay of the district court's ruling, enabling Governor Jerry Brown to order same-sex marriages to resume.

The passage of Prop 8 received widespread media coverage over its effect on the concurrent 2008 presidential and congressional elections, as well as the pre-election effects Prop 8 had on California's reputation as a historically LGBTQ-friendly state and the same-sex marriage debate that had started after same-sex marriage was legalized in Massachusetts through a 2004 court decision. After the results were certified and same-sex marriages ceased, supporters of Prop 8 were targeted by opponents with actions ranging from some opponents disclosing supporter donations and boycotting proponents' businesses, to others threatening supporters with death and vandalizing churches.

A ballot proposal to formally repeal Prop 8 from California's constitution, called Proposition 3, was passed by the California State Legislature in July 2023. The vote to formally repeal Prop 8 was passed by nearly 63% of voters in the 2024 election.

In 2000, the State of California adopted Proposition 22 which, as an ordinary statute, forbade recognition or licensing of same-sex marriages in the state. During February and March 2004, San Francisco Mayor Gavin Newsom directed the licensing of same-sex marriages on the basis of the state's equal protection clause, prompted also by recent events including George W. Bush's proposed constitutional ban, a possible legal case by Campaign for California Families (CCF), and a Supreme Court of Massachusetts ruling deeming same-sex marriage bans unconstitutional and permitting them from May 2004. While only lasting a month before being overruled, this was supported by other cities such as San Jose, gained global attention, and led to the case In re Marriage Cases, in which Proposition 22 was found (San Francisco County Superior Court, March 14, 2005) and confirmed upon appeal (California Supreme Court, May 15, 2008) to be unconstitutional.

Proposition 8 was created by opponents of same-sex marriage prior to the final ruling on In re Marriage Cases as a voter ballot initiative, and voted on at the time of the November 2008 elections. Its wording was precisely the same as Proposition 22, which as an ordinary statute, had been invalidated in 2008, but by re-positioning it as a state constitutional amendment rather than a legislative statute, it was able to circumvent the ruling from In re Marriage Cases. The proposition did not affect domestic partnerships in California, nor (following subsequent legal rulings) did it reverse same-sex marriages that had been performed during the interim period May to November 2008 (i.e. after In re Marriage Cases but before Proposition 8).