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.

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.