The first same-sex marriages in England and Wales are performed.

Same-sex marriage is legal in all parts of the United Kingdom. As marriage is a devolved legislative matter, different parts of the UK legalised at different times; it has been recognised and performed in England and Wales since March 2014, in Scotland since December 2014, and in Northern Ireland since January 2020.

Legislation to allow same-sex marriage in England and Wales was passed by the Parliament of the United Kingdom in July 2013 and took effect on 13 March 2014. The first same-sex marriages took place on 29 March 2014.

Legislation to allow same-sex marriage in Scotland was passed by the Scottish Parliament in February 2014 and took effect on 16 December 2014. The first same-sex marriage ceremonies for same-sex couples previously in civil partnerships occurred on 16 December. The first same-sex marriage ceremonies for couples not in civil partnerships occurred on 31 December 2014.

Legislation to allow same-sex marriage in Northern Ireland was passed by the Parliament of the United Kingdom in July 2019 and took effect on 13 January 2020. The first same-sex marriage ceremony took place on 11 February 2020.Same-sex marriage is legal in eight of the fourteen British Overseas Territories. It has been recognised in South Georgia and the South Sandwich Islands since 2014, Akrotiri and Dhekelia and the British Indian Ocean Territory (for UK military personnel only) since 3 June 2014, the Pitcairn Islands since 14 May 2015, the British Antarctic Territory since 13 October 2016, Gibraltar since 15 December 2016, the Falkland Islands since 29 April 2017 and Saint Helena, Ascension and Tristan da Cunha since 20 December 2017. Same-sex marriage was formerly legal in Bermuda from 2017-2022, but domestic partnerships are available to same-sex couples since 1 June 2018. Civil partnerships were legalised in the Cayman Islands on 4 September 2020.

Same-sex marriage is legal in the Crown dependencies. It has been recognised and performed in the Isle of Man since 22 July 2016, in Jersey since 1 July 2018, and in the Bailiwick of Guernsey at different times: in Guernsey since 2 May 2017, in Alderney since 14 June 2018, and in Sark since 23 April 2020.