General Alfred John Gardyne Drummond de Chastelain (born July 30, 1937) is a Canadian soldier and diplomat. Born a British subject in 1937, he emigrated to Canada in 1955 and was naturalized in 1962.

De Chastelain was born in Bucharest, Romania. His father was Alfred Gardyne de Chastelain, a Scottish oil engineer then working in Bucharest for British Petroleum, and Marion Elizabeth (Walsh) de Chastelain, an American woman. At the outbreak of World War II, both his parents worked secretly as spies for the British government.

De Chastelain was educated at Fettes College in Edinburgh and at the Royal Military College of Canada, from where he graduated in 1960 with a Bachelor of Arts degree in History and a Commission in the Princess Patricia's Canadian Light Infantry (PPCLI). Serving on regimental duty in Canada, Germany and Cyprus, de Chastelain attended the British Army Staff College in Camberley in 1966 and was Commanding Officer of the Second Battalion PPCLI from 1970 to 1972.

As a Colonel he commanded the Canadian Forces Base at Montreal for a two year period ending with the 1976 Olympic Games in that city. He was also Deputy Chief of Staff of the United Nations Force in Cyprus (UNFICYP)and Commander of the Canadian Contingent there. As a Brigadier-General he was successively Commandant of the Royal Military College of Canada, Commander of the 4th Canadian Mechanized Brigade Group in Lahr, Federal Republic of Germany, and Director General Land Doctrine and Operations at National Defence Headquarters in Ottawa.

As a Major-General he was Deputy Commander of the Canadian Land Force (Force Mobile Command) and Commander of the Mobile Command Division. As a Lieutenant-General he was Assistant Deputy Minister for Personnel, and then Vice-Chief of the Defence Staff. In 1989 he was promoted General and appointed Chief of the Defence Staff. In 1993 he transferred to the Reserves and was appointed Canada's Ambassador to the United States of America. In 1994 he was recalled to active duty and re-appointed Chief of the Defence Staff, from which post he retired in December 1995.

Since November 1995 de Chastelain has been involved in the Northern Ireland peace process, and is the Chairman of the Independent International Commission on Decommissioning.

In 1985 de Chastelain was appointed Commander of the Order of Military Merit and in 1991 Commander of the Order of St John; in 1993 he received the Commendation Medal of Merit and Honour of Greece, and was appointed Officer of the Order of Canada; in 1995 he was appointed Commander of the Legion of Merit (U.S.A.), and in 1999 he was made a Companion of Honour.

He has an honourary Doctor of Military Science degree from the Royal Military College of Canada, and an honourary Doctor of Laws (Conflict Resolution) degree from Royal Roads University in British Columbia.

John de Chastelain is married, and he and his wife have two children and five grandchildren.