Same-sex marriage, also known as gay marriage or same-gender marriage, is the marriage of two people of the same legal sex or gender. As of 2025,^[update] marriage between same-sex couples is legally performed and recognized in 38 countries, with a total population of 1.5 billion people (20% of the world's population). The most recent jurisdiction to legalize same-sex marriage is Thailand.

Same-sex marriage is legally recognized in a large majority of the world's developed countries; notable exceptions are Italy, Japan, South Korea, and the Czech Republic. Adoption rights are not necessarily covered, though most states with same-sex marriage allow those couples to jointly adopt as other married couples can. Some countries, such as Nigeria and Russia, restrict advocacy for same-sex marriage. A few of these are among the 35 countries (as of 2023) that constitutionally define marriage to prevent marriage between couples of the same sex, with most of those provisions enacted in recent decades as a preventative measure. Other countries have constitutionally mandated Islamic law, which is generally interpreted as prohibiting marriage between same-sex couples.^{[citation needed]} In six of the former and most of the latter, homosexuality itself is criminalized.

There are records of marriage between men dating back to the first century. Michael McConnell and Jack Baker are the first same sex couple in modern recorded history known to obtain a marriage license, have their marriage solemnized, which occurred on September 3, 1971, in Minnesota, and have it legally recognized by any form of government. The first law providing for marriage equality between same-sex and opposite-sex couples was passed in the continental Netherlands in 2000 and took effect on 1 April 2001. The application of marriage law equally to same-sex and opposite-sex couples has varied by jurisdiction, and has come about through legislative change to marriage law, court rulings based on constitutional guarantees of equality, recognition that marriage of same-sex couples is allowed by existing marriage law, and by direct popular vote, such as through referendums and initiatives. The most prominent supporters of same-sex marriage are the world's major medical and scientific communities, human rights and civil rights organizations, and some progressive religious groups, while its most prominent opponents are from conservative religious groups (some of which nonetheless support same-sex civil unions providing legal protections for same-sex couples). Polls consistently show continually rising support for the recognition of same-sex marriage in all developed democracies and in many developing countries.

Scientific studies show that the financial, psychological, and physical well-being of gay people is enhanced by marriage, and that the children of same-sex parents benefit from being raised by married same-sex couples within a marital union that is recognized by law and supported by societal institutions. At the same time, no harm is done to the institution of marriage among heterosexuals. Social science research indicates that the exclusion of same-sex couples from marriage stigmatizes and invites public discrimination against gay and lesbian people, with research repudiating the notion that either civilization or viable social orders depend upon restricting marriage to heterosexuals. Same-sex marriage can provide those in committed same-sex relationships with relevant government services and make financial demands on them comparable to that required of those in opposite-sex marriages, and also gives them legal protections such as inheritance and hospital visitation rights. Opposition is often based on religious teachings, such as the view that marriage is meant to be between men and women, and that procreation is the natural goal of marriage. Other forms of opposition are based on claims such as that homosexuality is unnatural and abnormal, that the recognition of same-sex unions will promote homosexuality in society, and that children are better off when raised by opposite-sex couples. These claims are refuted by scientific studies, which show that homosexuality is a natural and normal variation in human sexuality, that sexual orientation is not a choice, and that children of same-sex couples fare just as well as the children of opposite-sex couples.

Some proponents of the legal recognition of same-sex marriage—such as Marriage Equality USA (founded in 1998), Freedom to Marry (founded in 2003), Canadians for Equal Marriage, and Marriage for All Japan—used the terms marriage equality and equal marriage to signal that their goal was for same-sex marriage to be recognized on equal ground with opposite-sex marriage. The Associated Press recommends the use of same-sex marriage over gay marriage. A further move to more inclusive language has been the shift from same-sex relationship to same-gender relationship, and similarly to same-gender marriage to not restrict individuals within the relationship by sexual orientation, as gender identity is a spectrum and noting that historical terms excluded members of the LGBTQ+ community and were forms of bisexual or transgender erasure.

Anthropologists have struggled to determine a definition of marriage that absorbs commonalities of the social construct across cultures around the world. Many proposed definitions have been criticized for failing to recognize the existence of same-sex marriage in some cultures, including those of more than 30 African peoples, such as the Kikuyu and Nuer.

With several countries revising their marriage laws to recognize same-sex couples in the 21st century, all major English dictionaries have revised their definition of the word marriage to either drop gender specifications or supplement them with secondary definitions to include gender-neutral language or explicit recognition of same-sex unions. The Oxford English Dictionary has recognized same-sex marriage since 2000.

Opponents of same-sex marriage who want marriage to be restricted to pairings of a man and a woman, such as the Church of Jesus Christ of Latter-day Saints, the Catholic Church, and the Southern Baptist Convention, use the term traditional marriage to mean opposite-sex marriage.