John Harrison (March 24, 1693 – March 24, 1776) is a self-educated English carpenter and later a clockmaker.

He invents the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionizing and extending the possibility of safe long distance sea travel in the Age of Sail.

The problem is considered so intractable that the British Parliament had offered a prize of £20,000 (comparable to £2.87 million in modern currency) for the solution.

Harrison comes 39th in the BBC's 2002 public poll of the 100 Greatest Britons.