There have been several well-known people called John Day:

• John Day (Old Testament scholar), D.D., Professor of Old Testament Studies in the University of Oxford, Fellow, Tutor in Theology, and Dean of Degrees, Lady Margaret Hall
• John Day (fl. 1497), English merchant trading with Spain, who provided the most detailed account of John Cabot's voyage of 1497
• John Day (printer) (c.1522–1584), English Protestant printer, printer of John Foxe's Actes and Monuments
• John Day (dramatist) (1574–c.1640), English dramatist of the Elizabethan and Jacobian periods
• John Day (1738–1808), Fellow of the Royal Society
• Sir John Charles Day (1826-1908), English Judge, Privy Councillor, art collector
• Sir John Day (RAF officer), senior RAF officer
• John Day (born 1955), Member for Darling Range in the Western Australian Legislative Assembly
• John Day (baseball manager) (1847–1926), American Major League Baseball manager

There are several places in the U.S. state of Oregon named John Day:

• John Day, Oregon, a town
• John Day River, a tributary of the Columbia River in northeastern Oregon
• John Day River (northwestern Oregon), a different tributary of the Columbia River in northwestern Oregon
• John Day Fossil Beds National Monument
• John Day Dam on the Columbia River, United States