William Day may refer to:
- William H. Day (1825–1900), U.S. Black abolitionist
- William Day (bishop) (1529–1596), English clergyman
- William Day (divine) (died 1684), English clergyman
- William Day (sea captain) (1715–1797), Massachusetts naval captain during the American Revolutionary War
- William Day (architect), Irish architect from County Wexford
- William Day (horseman) (1823–1908), British racehorse trainer
- William Day (lithographer) (1797–1845), British lithographer and watercolour artist
- William Day (Australian politician) (fl. 1861), member of the New South Wales Legislative Council
- William J. Day (1876–1950), judge from Massachusetts
- William Louis Day (1876–1936), American judge
- William Peyton Day (1886–1966), architect and co-founder of architectural firm Weeks and Day with Charles Peter Weeks
- William R. Day (1849–1923), American diplomat and Supreme Court Justice
- William S. Day (1923–1984), American politician in the state of Washington
- Will Day, former chair of the UK Sustainable Development Commission
- Will Day (born 2001), Australian rules footballer
- Bill Day (cartoonist), American cartoonist
- Bill Day (baseball) (1867–1923), Major League Baseball player
- Bill Day (politician), Virginia
- Bill Day (alpine skier) (born 1934), skied for Australia at the 1952 Winter Olympics
- Bill Day (filmmaker), American documentary filmmaker