William Hay may refer to:
- William Hay (died 1664) (1594–1664), English politician who sat in the House of Commons variously between 1641 and 1660
- William Hay (architect) (1818–1888), Scottish architect
- William Hay (Australian politician) (1816–1908)
- William Hay (author) (1875–1945), Australian author and essayist
- William Hay (bishop) (1647–1707), Scottish bishop
- William Hay (Seaford MP) (1695–1755), British Member of Parliament for Seaford, 1734–1755
- William Hay (police commissioner) (1794–1855), Joint Commissioner of the London Metropolitan Police, 1850–1855
- William Hay, 10th Marquess of Tweeddale (1826–1911), British politician, Member of Parliament for Taunton and Haddington
- William Hay, 11th Marquess of Tweeddale (1884–1967), Scottish aristocrat, land owner and soldier
- Willie Hay, Baron Hay of Ballyore (born 1950), Northern Irish peer and former Member of the Northern Ireland Assembly
- William Hay, 1st Earl of Erroll (1423–1462), Scottish peer
- William Hay, 4th Earl of Kinnoull (died 1677), Scottish peer
- William Hay, 6th Earl of Kinnoull (died 1709), Scottish peer
- William Hay, 15th Earl of Kinnoull (1935–2013), peer, surveyor and farmer
- William Hay, 3rd Earl of Erroll (1449–1507), Scottish peer
- William Hay, 4th Earl of Erroll (1449–1513), Scottish peer and soldier
- William Hay, 5th Earl of Erroll (1495–1522), Scottish peer and statesman
- William Hay, 6th Earl of Erroll (1521–1541), Scottish peer
- William Hay, 10th Earl of Erroll (1590s–1636), Scottish nobleman
- William Hay, 17th Earl of Erroll (1772–1819), Scottish peer
- William Hay, 18th Earl of Erroll (1801–1846), Scottish peer
- William Hay, 19th Earl of Erroll (1823–1891), Scottish peer
- William Hay, 6th Lord Hay of Yester (died 1591), Scottish nobleman and courtier
- William Hay (English cricketer) (1849–1925), English cricketer
- William Arthur Hay (1873–1945), Australian clergyman and cricketer
- William Howard Hay (1866–1940), American doctor, author, lecturer and conceiver of the food combining diet
- William Perry Hay (1872–1947), American zoologist
- William Robert Hay (1761–1839), British barrister, cleric and magistrate
- Rupert Hay (William Rupert Hay, 1893–1962), British Indian Army officer and administrator in British India
- William Delisle Hay (1853–?), British author
- William H. Hay (1860–1946), United States Army officer
- William Walter Hay (1908–1998), American civil engineer and University of Illinois professor of railway engineering
- William W. Hay (1934–2022), geologist and paleoclimatologist with the University of Colorado