Marcellus I, Pope
- Long hagiography of Pope St. Marcellus I.
- http://www.saintpatrickdc.org/ss/0116.htm#marc

Marcellus I
- Brief biography paints St. Marcellus I as a rigorist who refused to accept the penitence of the lapsed, and says that Christian Rome rioted against him.
- http://www2.evansville.edu/ecoleweb/glossary/marcellusi.html

St. Marcellus I
- Brief account of his pontificate, from "Popes Through the Ages" by Joseph Brusher.
- http://www.cfpeople.org/Books/Pope/POPEp30.htm

Pope St. Marcellus I
- After a vacancy in office following the death of Pope St. Marcellinus, was elected to the papacy in 308. Fairly lengthy biographical article in the Catholic Encyclopedia.
- http://www.newadvent.org/cathen/09640b.htm