| Joseph-William Gagnon | |
| Office: | Member of the Legislative Assembly of Quebec for Maskinongé |
| Predecessor: | Rodolphe Tourville |
| Successor: | Louis-Joseph Thisdel |
| Term Start: | 1927 |
| Term End: | 1929 |
| Birth Date: | 15 February 1879 |
| Birth Place: | Yamachiche, Quebec |
| Death Place: | Louiseville, Quebec |
| Party: | Liberal |
Joseph-William Gagnon (February 15, 1879 - December 17, 1929) was a politician in the Quebec, Canada. He served as Member of the Legislative Assembly.
He was born on February 15, 1879, in Yamachiche, Mauricie.When just 5 years old, Gagnon's father William was killed along with several others in an avalanche during an organised hike in the mountains of Quebec. This strengthened Gagnon's relationship with his mother Elizabeth Darcy-Gagnon. In his memoirs, Gagnon is dedicated his political achievements to "That most admirable woman, whom I loved so dearly".
Gagnon served as Mayor of Louiseville from 1922 to 1926.
Gagnon ran as a Liberal candidate to the Legislative Assembly of Quebec in 1927 in the district of Maskinongé and won.
He died in office on December 17, 1929.