Sir William James Lynton Blair (born 31 March 1950) is a British retired judge. He was previously a Queen's Counsel at London barristers' chambers 3 Verulam Buildings, specialising in domestic and international banking and finance law. He is the elder brother of Sir Tony Blair, the former British prime minister.
 Professor of Financial Law and Ethics at the Centre for Commercial Law Studies, Queen Mary University of London, he served as a High Court Judge in England and Wales for nearly ten years, and was Judge in Charge of the Commercial Court from 2016. 
 In 2017, Blair rejoined 3 Verulam Buildings as an Associate Member and international arbitrator, and is now a member of 3VB's [International Advisory and Dispute Resolution Unit (IADRU).
 He is the Chair of the Bank of England’s Enforcement Decision Making Committee (EDMC) and in 2018 was appointed to the International Commercial Expert Committee of the Supreme People’s Court of the People’s Republic of China.
 Blair became a Queen's Counsel in 1994, appearing and advising in many domestic and international disputes and matters particularly in the financial field. 
 He was appointed a recorder in 1998.[citation needed] He became a deputy High Court judge in 2003 and was in the same year admitted as a barrister of the Eastern Caribbean Supreme Court in the Territory of the Virgin Islands.[citation needed]
 On 4 February 2008 it was announced that the Queen had approved his appointment as a High Court judge, assigned to the Queen's Bench Division. He was sworn in on 28 February. In 2016 he became judge in charge of the Commercial Court.
 He became a member of London's Financial Markets Law Committee in 2008.
 In 2012, he became the first President of the Board of Appeal of the European Supervisory Authorities, serving until 2019.
 He is or has been a visiting professor of law at the London School of Economics, Peking University Law School, East China University of Politics and Law and the University of Hong Kong. He has also been, since 1999, an Honorary Fellow of the Society for Advanced Legal Studies (1997), and an academic adviser to the Asian Institute of International Financial Law.
 From 2001 to 2008 he served, part-time, as a chairman of the Financial Services and Markets Tribunal (Finsmat) and from 2003 to 2005 he was chairman of the Commercial Bar Association (Combar). He is also chairman of the International Monetary Law Committee of the International Law Association (2004-2021), and of the Qatar Financial Centre Regulatory Tribunal.
 Along with Lord Woolf, the former Lord Chief Justice of England and Wales, he served as the Co-Convener of the inaugural Qatar Law Forum of Global Leaders in Law, held in Doha, Qatar, from 29–31 May 2009.[citation needed]
 He retired as a High Court Judge on 1 December 2017 to take up a position as Professor of Financial Law and Ethics at the Centre for Commercial Law Studies, Queen Mary University of London.
 Since 2018, as well as arbitration, he has also sat part-time as a Deputy Judge of the High Court of Hong Kong SAR (where he is given a Chinese name "貝理賢" by the Hong Kong Judiciary), and in Qatar International Court. At the China International Commercial Court, he has spoken on legal issues.
 He is a member of the Ethics Committee of Digital Catapult’s AI Machine Intelligence Garage.