Guibert of Ravenna, d. 1100, Italian churchman, antipope (1080-1100) Clement III, b. Parma. As imperial chancellor of Italy (1057-63), he consistently supported the Holy Roman emperor's opposition to papal reform efforts, and he led the party that repudiated Pope Alexander II. Emperor Henry IV made him archbishop of Ravenna (1072), and Guibert continued to intrigue against Gregory VII. In 1080, after Gregory excommunicated the emperor, Guibert summoned a council that declared the pope deposed and chose Guibert to replace him. The antipope entered Rome in 1083 in Henry's train, was enthroned in 1084, and crowned the emperor. His name is sometimes spelled Wibert.