Alexander Boris de Pfeffel Johnson (born 19 June 1964) is a British politician and writer serving as Prime Minister of the United Kingdom and Leader of the Conservative Party since 2019. He was Secretary of State for Foreign and Commonwealth Affairs from 2016 to 2018 and Mayor of London from 2008 to 2016. Johnson has been Member of Parliament (MP) for Uxbridge and South Ruislip since 2015 and was previously MP for Henley from 2001 to 2008. He has been described as adhering to the ideology of one-nation conservatism.

Johnson was educated at Eton College and studied Classics at Balliol College, Oxford. He was elected President of the Oxford Union in 1986. In 1989, he became the Brussels correspondent, and later political columnist, for The Daily Telegraph, where his articles exerted a strong Eurosceptic influence on the British right-wing of politics. He was editor of The Spectator magazine from 1999 to 2005. After being elected to Parliament in 2001, Johnson was a shadow minister under Conservative leaders Michael Howard and David Cameron. In 2008, he was elected Mayor of London and resigned from the House of Commons; he was re-elected as mayor in 2012. In the 2015 election, Johnson was elected MP for Uxbridge and South Ruislip. The following year, he did not seek re-election as mayor. He became a prominent figure in the successful Vote Leave campaign for Brexit in the 2016 EU membership referendum. Theresa May appointed him foreign secretary after the referendum; he resigned the position two years later in protest at May's approach to Brexit and the Chequers Agreement.

After May resigned in 2019, Johnson was elected Conservative leader and appointed prime minister. He re-opened Brexit negotiations and in early September controversially prorogued Parliament; the Supreme Court ruled the action unlawful later that month. After agreeing to a revised Brexit withdrawal agreement with the EU, which replaced the Irish backstop with a new Northern Ireland Protocol, but failing to win parliamentary support for the agreement, Johnson called a snap election for December 2019 in which he led the Conservative Party to victory with 43.6% of the vote, and the party's largest seat share since 1987. On 31 January 2020, the United Kingdom withdrew from the EU, entering into a transition period and trade negotiations leading to the EU–UK Trade and Cooperation Agreement. The COVID-19 pandemic had a major impact on the UK; his government responded with various emergency powers and introduced measures across society to mitigate its impact. Johnson initially reacted slowly to the outbreak and resisted introducing lockdown measures, although the vaccination programme was the world's first.

Johnson is considered a controversial figure in British politics. Supporters have praised him as humorous and entertaining, with an appeal stretching beyond traditional Conservative voters. Conversely, his critics have accused him of elitism, cronyism, and bigotry. Commentators have described his political style as opportunistic, populist or pragmatic.