William Allan may refer to:

• George William Allan (1822–1901), Canadian politician
• George William Allan (Manitoba politician) (1860–1940), Canadian politician
• William Allan (banker) (c. 1770–1853), Canadian banker, businessman and politician
• William Allan (English politician) (1837–1903), Liberal Party politician in the United Kingdom
• William Allan (Queensland politician) (1840–1901), pastoralist and politician in Queensland, Australia

• William Allan (footballer, born 1880) (1880–1965), Scottish goalkeeper for Falkirk, Rangers and Hibernian
• William Allan (footballer, born 1904) (1904–1969), Scottish defender for Hamilton and Motherwell
• Willie Allan (born 1942), Scottish footballer
• Wee Willie Allan (1930 - 2019), American dirt modified racing driver

• William Allan (British Army officer) (1832–1918)
• William Allan (classicist) (born 1970), Scottish classicist
• William Allan (geneticist) (1881–1943), American physician and geneticist
• William Allan (painter) (1782–1850), Scottish historical painter
• William Allan (trade unionist) (1813–1874), British trade unionist
• William Evan Allan (1899–2005), Australian military figure, seeing service in WWI and WWII
• William Allan of Glen (1788–1868), Lord Provost of Edinburgh

• Bill Allen (disambiguation)
• Will Allen (disambiguation)
• William Allen (disambiguation)
• William FitzAlan, Lord of Oswestry (c. 1105–1160), Norman noble
• William Van Alen (1883–1954), American architect
• Norman Thomas William Allan, known as Bill, former Australian (N.S.W.) police commissioner