| Honorific-Prefix: | The Honourable |
| Léo Cadieux | |
| Office: | Canadian Ambassador to France |
| Primeminister: | Pierre Trudeau |
| Term Start: | 16 October 1970 |
| Term End: | 8 September 1975 |
| Predecessor: | Paul André Beaulieu |
| Successor: | Gérard Pelletier |
| Office1: | Minister of National Defence |
| Primeminister1: | Lester B. Pearson Pierre Trudeau |
| Term Start1: | 19 September 1967 |
| Term End1: | 16 September 1970 |
| Predecessor1: | Paul Hellyer |
| Successor1: | Charles Drury (acting) |
| Office2: | Associate Minister of National Defence |
| Primeminister2: | Lester B. Pearson |
| Term Start2: | 15 February 1965 |
| Term End2: | 18 September 1967 |
| Predecessor2: | Lucien Cardin |
| Successor2: | Vacant |
| Riding3: | Labelle |
| Parliament3: | Canadian |
| Term Start3: | 25 June 1968 |
| Term End3: | 5 October 1970 |
| Predecessor3: | Gaston Clermont |
| Successor3: | Maurice Dupras |
| Riding4: | Terrebonne |
| Parliament4: | Canadian |
| Term Start4: | 18 June 1962 |
| Term End4: | 24 June 1968 |
| Predecessor4: | Marcel Deschambault |
| Successor4: | Joseph-Roland Comtois |
| Birth Name: | Joseph Alphonse Léo Cadieux |
| Birth Date: | 28 May 1908 |
| Birth Place: | Saint-Jérôme, Quebec, Canada |
| Death Place: | Ottawa, Ontario, Canada |
| Children: | 1 |
| Party: | Liberal |
Léo Alphonse Joseph Cadieux (in French kadjø/; May 28, 1908 – May 11, 2005) was a Canadian politician.
A newspaper journalist and publisher who was born in Saint-Jérôme, Quebec, Cadieux was first elected to the House of Commons of Canada as the Liberal Member of Parliament for Terrebonne, Quebec in the 1962 election.
In 1965, he was appointed to the Cabinet by Prime Minister Lester Pearson as Associate Minister of National Defence. In 1967, he was promoted to Minister of National Defence (and the first Francophone to hold the post), and remained in that position under Pearson and then Pierre Trudeau until he retired from politics in 1970.
On his retirement from Parliament, he was appointed Canada's Ambassador to France. He remained Canada's envoy until 1975.
During Cadieux's tenure as Defence Minister, Canada cut its troop commitment to Europe from 10,000 to 5,000 troops and ended Canada's commitment to send re-enforcements to the North Atlantic Treaty Organization's Central Front following a review of Canadian defence priorities. In Cabinet debates on Canada's attitude towards nuclear deterrence, Cadieux argued in support of the doctrine. He also oversaw the reorganization of the Canadian Emergency Measures Organization, Canada's civil defence agency.
In 1974, he was made an Officer of the Order of Canada.