Leo of Ostia

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 twelfth 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 Chronicon Monasterii Casinensis, usually called the Montecassino Chronicle in English. 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.