Marc Lalonde OC, PC (born July 26 1929) is a retired Canadian politician and Cabinet minister.

Marc Lalonde obtained a Master of Laws degree from the Université de Montréal, as well as a Master's degree from Oxford University and a Diplôme d'études supérieures en droit (D.E.S.D) from the University of Ottawa. In 1959, he worked in Ottawa as a a special advisor to Justice Minister E. Davie Fulton. He then went to Montreal to practice law until 1967 when he returned to Ottawa to work as an advisor in the Prime Minister's Office under Lester Pearson. Lalonde remained when Pierre Trudeau became Prime Minister of Canada in 1968 serving as Principal Secretary until the 1972 Canadian election when, at Trudeau's urging, he ran for a seat in the Canadian House of Commons. Elected as the Liberal MP for the riding of Outremont, Lalonde immediately joined the Cabinet as Minister of National Health and Welfare. A staunch federalist, he was also one of Trudeau's chief advisors on the situation in Quebec, taking the position of Minister of State on federal-provincial relations in the wake of the Parti Quebecois' victory in the 1976 Quebec election. Lalonde then served as Minister of Justice from 1978 until the government's defeat in the 1979 Canadian election.

When the Liberals returned to power in the 1980 Canadian election, Lalonde became Minister of Energy and instituted the National Energy Policy which became intensely unpopular in Alberta. From 1982 until 1984 he served as Minister of Finance instituting a limited program of informal wage and price controls in an effort to reduce inflation.

Lalonde remained Finance Minister when John Turner succeeded Trudeau as Prime Minister in 1984 but did not run in the 1984 Canadian election.

In 1989 he was made an Officer of the Order of Canada. In 2004 he was inducted into the Canadian Medical Hall of Fame.

In the 1990s he served as an ad hoc judge at the International Court of Justice and has also represented Canada in various trade disputes. He is a practising lawyer with the firm of Stikeman Elliott LLP in Montreal and is returning to politics in 2005, as prime minister Paul Martin named him Co-president of the electoral campaign in Québec, with Brigitte Legault, the President of the Young Liberals of Canada (Québec) at his side.