| Honorific-Prefix: | The Honourable |
| Onésime Gagnon | |
| Order: | 20th |
| Office: | Lieutenant Governor of Quebec |
| Predecessor: | Gaspard Fauteux |
| Successor: | Paul Comtois |
| Term Start: | February 14, 1958 |
| Term End: | September 30, 1961 |
| Office2: | Member of the Legislative Assembly of Quebec for Matane |
| Predecessor2: | Joseph-Arthur Bergeron |
| Successor2: | Benoît Gaboury |
| Term Start2: | August 17, 1936 |
| Term End2: | February 14, 1958 |
| Constituency Mp3: | Dorchester |
| Parliament3: | Canadian |
| Predecessor3: | Lucien Cannon |
| Successor3: | Léonard Tremblay |
| Term Start3: | July 28, 1930 |
| Term End3: | October 14, 1935 |
| Birth Date: | 23 October 1888 |
| Death Place: | Bois-de-Coulonge, Sillery, Quebec |
| Nationality: | Canadian |
| Party: | Federal:Provincial: |
| Cabinet: | Federal:Provincial: |
Onésime Gagnon (October 23, 1888 – September 30, 1961) was a Canadian politician who served as the 20th Lieutenant Governor of Québec.
He was born in Saint-Léon-de-Standon, Quebec, on October 23, 1888, and was the son of Onésime Gagnon and Julie Morin. He was a Rhodes scholar and was called to the Quebec Bar in 1912. From 1942 to 1958, he was a Professor in the Faculty of Law at Université Laval.
In 1930, he was elected to the House of Commons of Canada for the riding of Dorchester. A Conservative, he was re-elected in 1935. In 1935, he was a Minister without Portfolio in the cabinet of R. B. Bennett.
Gagnon was a leadership candidate at the Conservative Party of Quebec convention, held in Sherbrooke on October 4 and 5, 1933. He was defeated by Maurice Duplessis with 28% of the delegates.
In 1936, he was elected to the Legislative Assembly of Quebec and became the Union Nationale Member for the riding of Matane. He was appointed Minister in the Cabinet of Maurice Duplessis, serving as Minister of Fisheries from 1936 to 1939 and as Treasurer from 1944 to 1958.
He was re-elected in 1939, 1944, 1948, 1952, and 1956.
He resigned in 1958 to accept the office of Lieutenant-Governor of Quebec and served until his death.
Gagnon died on September 30, 1961.