Fiesco, Guglielmo an Italian prelate, was born in Genoa, and was the nephew of pope Innocent IV, who made him, in December 1244, cardinal-deacon, with the title of St. Eustachius. The same pontiff gave him the protectorate of the Augustinians, and placed him at the head of some troops in 1254, to operate against France. Guglielmo came back to Rome after the death of his uncle, and took part at the election of' pope Alexander IV, on December 12 of that year. He died in 1256, and was buried in the Church of San Lorenzo. See Hoefer, Nouv. Biog. Generale, s.v.