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) (1934–2024), Australian Olympic skier
• Bill Day (filmmaker), American documentary filmmaker