| Honorific Prefix: | The Reverend |
| Lewis J. Duncan | |
| Office: | Mayor of Butte |
| Term Start: | May 1, 1911 |
| Term End: | October 6, 1914 |
| Predecessor: | Charles P. Nevin |
| Successor: | Clarence A. Smith |
| Birth Name: | Lewis Johnstone Duncan |
| Birth Date: | 4 May 1857 |
| Birth Place: | St. Louis, Missouri, U.S. |
| Death Place: | Rochester, Minnesota, U.S. |
| Known For: | First Socialist mayor of Butte |
| Education: | Hanover College |
| Occupation: | Unitarian minister |
| Party: | Socialist |
| Otherparty: | Nonpartisan League |
Lewis Johnstone Duncan (May 4, 1857 - January 26, 1936) was an American Unitarian minister and politician who served two terms as mayor of Butte, Montana.[1] He was Butte's first Socialist mayor,[2] and the first to be re-elected.[3] He was recalled from office in 1914.[4]