John Hadley (disambiguation)
John Hadley (1682–1744) was a British mathematician and inventor of the octant. John Hadley may also refer to:

John Hadley (chemist) (1731–1764), British chemist and physician
John Hadley (Mayor of London) (fl.1300s), Mayor and MP for London, father-in-law of William Pecche
John Hadley (philosopher) (born 1966), Australian ethicist
John V. Hadley (c. 1839–1842–1915), Justice of the Indiana Supreme Court