Leo Marsicanus (meaning "of the Marsi") or Ostiensis (meaning "of Ostia"), also known as Leone dei Conti di Marsi (1046, Marsica – 1115/7, Ostia), was a nobleman and monk of Monte Cassino around 1061 and Italian cardinal from the 12th century.

In Monte Cassino, he became a friend of Desiderius of Benevento, later Pope Victor III, and it was to him that Leo dedicated his most famous work as an historian and chronicler, being a librarian: the Chronica Monasterii Casinensis. The chronicler depends largely on Amatus' earlier work, but also on oral traditions and other archives. Leo finished it at 1075; it is continued by other monastic librarian Peter the Deacon.

Pope Urban II created him cardinal deacon in 1088 with the deaconry of Ss. Vito e Modesto. In 1101, Pope Paschal II promoted him cardinal-bishop of Ostia. In 1105, he was appointed cardinal - bishop of Velletri until his death.

• Norwich, John Julius. The Normans in the South 1016-1130. Longmans: London, 1967.
•  Herbermann, Charles, ed. (1913). "Ostiensis". Catholic Encyclopedia. New York: Robert Appleton Company.
• Miranda, Salvador. "LEONE, O.S.B.Cas. (?-1116)". The Cardinals of the Holy Roman Church. Florida International University. OCLC 53276621.

• Chronicon Monasterii Casinensis (Leo Marsicanus, Cardinalis: Chronica Monasterii Casinensis)

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