| John Plummer Marcellus | |
| Birth Date: | 26 July 1838 |
| Birth Place: | Morrisburg, Upper Canada |
| Death Place: | Pincher Creek, Alberta |
| Office: | Member of the Legislative Assembly of Alberta |
| Constituency: | Pincher Creek |
| Term Start: | 1905 |
| Term End: | 1909 |
| Successor: | David Warnock |
| Party: | Liberal |
| Occupation: | rancher, farmer |
John Plummer Marcellus (July 26, 1838 – May 29, 1932) was a Canadian politician and rancher from Alberta.
He was born at Morrisburg, Upper Canada. Marcellus married his wife Maria Barkley in 1871 at Dunbar, Ontario.
Marcellus ran for the Alberta legislature in the 1905 Alberta general election. He won a hotly contested 3-way race in the Pincher Creek electoral district in rural southeast Alberta. He served one term as a backbencher in the Legislative Assembly before retiring in 1909. He died in 1932 at Pincher Creek.