John Day, John Daye or Jon Day may refer to:

• John Day (Nova Scotia legislator) (died 1775), merchant and politician in Nova Scotia
• John Day Jr. (died 1792), soldier and political figure in Nova Scotia
• John Day (Liberian judge) (1797–1859), Liberian politician and judge
• John Charles Day (1826–1908), English judge
• John Adam Day (1901–1966), politician in Devon, England
• John Day (Indiana politician) (born 1937), Democratic member of the Indiana House of Representatives
• John Day (Australian politician) (born 1955), Western Australian politician
• John Day, Transport Workers Union of America Local 100 Vice President

• John Barham Day (1793–1860), English jockey and trainer
• John Day (cricketer, born 1812), English cricketer
• John Day (horseman) (1819–1883), English jockey and trainer
• John B. Day (1847–1925), manager of the New York Giants in 1899
• John Day (jockey) (1856–1885) Australian champion pedestrian and 1870 Melbourne Cup winner
• John Day (cricketer, born 1881) (1881–1949), English cricketer

• John Day (merchant) (fl. 1497–98), English merchant, author of a letter referring to existence of lost book Inventio Fortunata
• John Day (printer) (c. 1522–1584), English Protestant printer, also known as John Daye
• John Day (dramatist) (1574–c. 1638), English dramatist
• John Day (carpenter) (c. 1740–1774), first recorded death in a submarine
• John Day (trapper) (c. 1770–1820), American hunter and trapper
• John Day (architect) (fl. 1798–1802), Irish architect from County Wexford
• John Day (priest) (1802–1879), clergyman in the Church of Ireland
• John Day (botanist) (1824–1888), English orchid collector and illustrator
• John Medway Day (1838–1905), Australian journalist and activist
• John Other Day (fl. 1862–1912), Native American who sought peace between Indian tribes and white settlers
• John H. Day (1909–1989), South African marine biologist and invertebrate zoologist
• John A. Day (1913–2008), American meteorologist, educator and sky-watching evangelist
• John Day (historian) (1924–2003), American historian
• John Day (computer scientist) (born 1947), ARPANET pioneer and early RFC contributor
• John Day (RAF officer) (1947–2024), British air marshal
• Johnny Daye (1948–2017), American soul music singer
• John Day (biblical scholar) (born 1948), professor of Old Testament Studies
• Jon Day (born 1954), British civil servant
• Jon Day (born 1962), member of The Charlatans
• Jon Day (writer) (fl. 2010s), British writer, critic and academic

• John Day, Oregon, a city
• John Day River, a tributary of the Columbia River in northeastern Oregon
• John Day River (northwestern Oregon), a different tributary of the Columbia River

• John Day (film), a 2013 Indian film
• John Day Company, a New York publisher founded in 1926

• John Day Dam, a dam on the Columbia River
• John Day Fossil Beds National Monument