The 2016 United Kingdom European Union membership referendum, commonly referred to as the EU referendum or the Brexit referendum, was a referendum that took place on 23 June 2016 in the United Kingdom (UK) and Gibraltar under the provisions of the European Union Referendum Act 2015 to ask the electorate whether the country should continue to remain a member of, or leave, the European Union (EU). The result was a vote in favour of leaving the EU, triggering calls to begin the process of the country's withdrawal from the EU commonly termed "Brexit".

Since 1973, the UK had been a member state of the EU and its predecessor the European Communities (principally the European Economic Community), along with other international bodies. The constitutional implications of membership for the UK became a topic of debate domestically particularly regarding sovereignty. A referendum on continued membership of the European Communities (EC) to try and settle the issue was held in 1975, with 67% of voters approving continued membership. Between 1975 and 2016 as European integration deepened, subsequent EC/EU treaties and agreements were ratified by the UK Parliament but without any public approval. Following the Conservative Party's victory at the 2015 general election as a main manifesto pledge, the legal basis for the EU referendum was established through the European Union Referendum Act 2015. Prime Minister David Cameron also oversaw a renegotiation of the terms of EU membership, intending to implement these changes in the event of a Remain result. The referendum was legally non-binding due to the ancient principle of parliamentary sovereignty, although the government promised to implement the result.

Official campaigning took place between 15 April and 23 June 2016. The official group for remaining in the EU was Britain Stronger in Europe while Vote Leave was the official group endorsing leaving. Other campaign groups, political parties, businesses, trade unions, newspapers and prominent individuals were also involved, with both sides having supporters from across the political spectrum. Parties in favour of remaining included Labour, the Liberal Democrats, the Scottish National Party, Plaid Cymru and the Green Party; while the UK Independence Party campaigned in favour of leaving; and the Conservative Party remained neutral. In spite of the Conservative and Labour Party's official positions, both parties allowed their Members of Parliament to publicly campaign for either side of the issue. Campaign issues included the costs and benefits of membership for the UK's economy, freedom of movement and migration. Several allegations of unlawful campaigning and Russian interference arose during and after the referendum.

The results recorded 51.9% of the votes cast being in favour of leaving. Most areas of England and Wales had a majority for Leave, and the majority of voters in Scotland, Northern Ireland, Greater London and Gibraltar chose Remain. Voter preference correlated with age, level of education and socioeconomic factors. The causes and reasoning of the Leave result have been the subject of analysis and commentary. Immediately after the result, financial markets reacted negatively worldwide, and Cameron announced that he would resign as prime minister and leader of the Conservative Party, which he did in July. The referendum prompted an array of international reactions. Jeremy Corbyn faced a Labour Party leadership challenge as a result of the referendum. In 2017, the UK gave formal notice of intent to withdraw from the EU, with the withdrawal being formalised in 2020.

The European Communities were formed in the 1950s – the European Coal and Steel Community (ECSC) in 1952, and the European Atomic Energy Community (EAEC or Euratom) and European Economic Community (EEC) in 1957. The EEC, the more ambitious of the three, came to be known as the "Common Market". The UK first applied to join them in 1961, but this was vetoed by France. A later application was successful, and the UK joined in 1973; two years later, a national referendum on continuing EC membership resulted in 67.2% voting "Yes" in favour of continued membership, on a 64.6% national turnout. However, no further referendums on the issue of the United Kingdom's relationship with Europe were held and successive British governments integrated further into the European project, which gained focus when the Maastricht Treaty established the European Union (EU) in 1993, which incorporated (and after the Lisbon Treaty, succeeded) the European Communities.

At the May 2012 NATO summit meeting, UK Prime Minister David Cameron, Foreign Secretary William Hague and Ed Llewellyn discussed the idea of using a European Union referendum as a concession to the Eurosceptic wing of the Conservative Party. On 20 June 2012, a three-clause private member's bill was introduced into the House of Commons by the then Eurosceptic MP Douglas Carswell to end the United Kingdom's EU membership and repeal the European Communities Act 1972, but without containing any commitment to the holding of any referendum. It received a second reading in a half-hour long debate in the chamber on 26 October 2012, but did not progress any further.

In January 2013, Cameron delivered the Bloomberg speech and promised that, should the Conservatives win a parliamentary majority at the 2015 general election, the British government would negotiate more favourable arrangements for continuing British membership of the EU, before holding a referendum on whether the UK should remain in or leave the EU. The Conservative Party published a draft EU Referendum Bill in May 2013, and outlined its plans for renegotiation followed by an in-out vote (i.e. a referendum giving options only of leaving and of remaining in under the current terms, or under new terms if these had become available), were the party to be re-elected in 2015. The draft Bill stated that the referendum had to be held no later than 31 December 2017.

The draft legislation was taken forward as a Private member's bill by Conservative MP James Wharton which was known as the European Union (Referendum) Bill 2013. The bill's First Reading in the House of Commons took place on 19 June 2013. Cameron was said by a spokesperson to be "very pleased" and would ensure the Bill was given "the full support of the Conservative Party".