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