Onésime Gagnon PC (French pronunciation: [ɔnezim ɡaɲɔ̃]; 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.

Gagnon was a leadership candidate at the Conservative Party of Quebec convention, held in Sherbrooke on October 4 and 5, 1933.

In 1936, he was elected to the Legislative Assembly of Quebec and became the Union Nationale Member for the riding of Matane.

He resigned in 1958 to accept the office of Lieutenant-Governor of Quebec and served until his death.