Marcel Cadieux, CC (June 17, 1915 – March 19, 1981) was a Canadian civil servant and diplomat.

He studied at the Collège André Grasset, obtained a Master's degree in law from the Université de Montréal,[1] and studied constitutional law at McGill University in Montreal.

Cadieux joined the Department of External Affairs in 1941,[2] served as senior adviser to Canadian members of the International Control Commission in Vietnam in 1954, and became the legal advisor to the Department of External Affairs in 1956.

[3][4] Cadieux served on the negotiating committee to determine maritime boundaries with the United States.

[5][1] He was Canada's first francophone Ambassador to the United States from 1970 to 1975,[6][7] and head of the Canadian Mission to the European Communities from 1975.