William Blair may refer to:

• William Blair (American politician) (1820–1880), member of the Wisconsin State Senate
• William Blair (Australian footballer) (1912–1960), Australian footballer and umpire
• William Blair (Ayrshire MP) (died 1841), member of parliament for Ayrshire 1829–1832
• William Blair (judge) (born 1950), British judge and KC; elder brother of U.K. prime minister Tony Blair
• William Blair (Nova Scotia politician) (1836–1919), farmer and political figure in Nova Scotia, Canada
• William Blair (Royal Navy officer) (1741–1782), Scottish sailor and captain in the Royal Navy
• William Blair (Scottish footballer) (born 1872), Scottish international footballer
• William Blair (surgeon) (1766–1822), British surgeon and writer on ciphers
• William D. Blair, Jr (1937–2006), reporter, State Department spokesman, president of The Nature Conservancy
• William Gourlay Blair (1890–1957), Canadian politician and physician
• William H. Blair (1821–1888), American Civil War Union Brevet Brigadier General
• William John Blair (1875–1943), Canadian engineer and politician
• William M. Blair (1884–1982), investment banker from Chicago
• William McCormick Blair Jr. (1916–2015), U.S. ambassador; son of William M. Blair
• William Newsham Blair (1841–1891), New Zealand engineer and surveyor
• William R. Blair (1874–1962), American scientist and U.S. Army officer
• William R.N. Blair (1915–1990), Canadian psychologist and academic administrator
• William W. Blair (1828–1896), leader in the Reorganized Church of Jesus Christ of Latter Day Saints
• William Blair Tennent (1898–1976), New Zealand politician and cabinet minister
• Willie Blair (born 1965), American baseball player

Companies

• William Blair & Company, a Chicago-based investment banking firm founded by William M. Blair