Same-sex marriage (also known as gay marriage) is marriage between two persons of the same biological sex or gender identity. Supporters of legal recognition for same-sex marriage typically refer to such recognition as marriage equality.

Since 2001, ten countries have begun allowing same-sex couples to marry nationwide: Argentina, Belgium, Canada, Iceland, the Netherlands, Norway, Portugal, Spain, South Africa, and Sweden. Same-sex marriages are also performed and recognized in Mexico City and parts of the United States. Some jurisdictions that do not perform same-sex marriages recognize same-sex marriages performed elsewhere: Israel, the Caribbean countries of the Kingdom of the Netherlands, parts of the United States, and all states of Mexico. Australia recognises same-sex marriages only by one partner changing their sex after marriage. As of 2012, proposals exist to introduce same-sex marriage in at least ten other countries. The introduction of same-sex marriage has varied by jurisdiction, resulting from legislative changes to marriage laws, court challenges based on constitutional guarantees of equality, or a combination of the two. In some countries, allowing same-sex couples to marry replaced a previous system of civil unions or registered partnerships.