Sir Anthony Charles Lynton Blair (born 6 May 1953) is a British former politician who served as Prime Minister of the United Kingdom from 1997 to 2007 and Leader of the Labour Party from 1994 to 2007. He served as Leader of the Opposition from 1994 to 1997, and had various shadow cabinet posts from 1987 to 1994. Blair was the Member of Parliament (MP) for Sedgefield from 1983 to 2007. He is the second longest serving prime minister in modern history after Margaret Thatcher, and is the longest serving Labour politician to have held the office.

Blair attended the independent school Fettes College, studied law at St John's College, Oxford, and later qualified as a barrister. He became involved in the Labour Party and was elected to the House of Commons in 1983 for the Sedgefield constituency in County Durham. As a backbencher, Blair supported moving the party to the political centre of British politics. He was appointed to Neil Kinnock's shadow cabinet in 1988 and was appointed Shadow Home Secretary by John Smith in 1992. Following the death of Smith in 1994, Blair won the 1994 Labour Party leadership election to succeed him. His tenure as leader began with a historic rebranding of the party, which began to use the campaign label New Labour. The name dates from a conference slogan first used by the party in 1994, later seen in a draft manifesto published in 1996 and titled New Labour, New Life for Britain, presented as the brand of a newly reformed party.

Blair was appointed prime minister after Labour won the 1997 general election, its largest landslide general election victory in history, becoming the youngest prime minister of the 20th century. During his first term, Blair enacted constitutional reforms, and significantly increased public spending on healthcare and education, while also introducing controversial market-based reforms in these areas. In addition, Blair saw the introduction of a minimum wage, tuition fees for higher education, constitutional reform such as devolution in Scotland and Wales, an extensive expansion of LGBT+ rights and significant progress in the Northern Ireland peace process with the passing of the landmark Good Friday Agreement. On foreign policy, Blair oversaw British interventions in Kosovo in 1999 and Sierra Leone in 2000, which were generally perceived as successful.

Blair was re-elected in a second landslide in 2001. Three months into his second term, Blair's premiership was shaped by the September 11 terrorist attacks in the United States, resulting in the start of the war on terror. Blair supported the foreign policy of the George W. Bush administration by ensuring that the British Armed Forces participated in the War in Afghanistan, to overthrow the Taliban, destroy al-Qaeda, and capture Osama bin Laden. In 2003, Blair supported the 2003 invasion of Iraq and had the British Armed Forces participate in the Iraq War, claiming that Saddam Hussein's regime possessed weapons of mass destruction (WMDs); no WMDs were ever found in Iraq.

Blair was re-elected for a third term with another landslide in 2005, in part thanks to the UK's strong economic performance, but with a substantially reduced majority, in part thanks to the UK's involvement in the Iraq War. Blair pushed for more systemic public sector reform during his his third term and brokered a settlement to restore powersharing to Northern Ireland. The Afghanistan and Iraq wars continued, and in 2006, Blair announced he would resign within a year. Blair resigned as Labour leader on 24 June 2007 and as prime minister on 27 June 2007, and was succeeded by Gordon Brown, his chancellor. After leaving office, Blair gave up his seat and was appointed Special Envoy of the Quartet on the Middle East, a diplomatic post which he held until 2015. He has been the executive chairman of the Tony Blair Institute for Global Change since 2016, and has made occasional political interventions. In 2009, Blair was awarded the Presidential Medal of Freedom by George W. Bush. He was knighted by Queen Elizabeth II in 2021.

At various points in his premiership, Blair was among both the most popular and most unpopular figures in UK history. As prime minister, he achieved the highest recorded approval ratings during his first few years in office, but also one of the lowest such ratings during and after the Iraq War. Blair had notable electoral successes and reforms, and he is usually rated as above average in historical rankings and public opinion of British prime ministers.