John Loudon is the name of:

John Loudon McAdam (23 September 1756 – 26 November 1836) was a Scottish civil engineer and road-builder. He was the inventor of "macadamisation ", an effective and economical method of constructing roads.

Jhr. John Loudon (18 March 1866, The Hague – 11 November 1955, Wassenaar ) was a Dutch top diplomat and politician who was Dutch Minister of Foreign Affairs during the First World War. He was a moderate liberal.