Antipope Clement III (b. 1029)
Guibert or Wibert of Ravenna (c. 1029 – 8 September 1100) was an Italian prelate, archbishop of Ravenna, who was elected pope in 1080 in opposition to Pope Gregory VII and took the name Clement III. Gregory was the leader of the movement in the church which opposed the traditional claim of European monarchs to control ecclesiastical appointments, and this was opposed by supporters of monarchical rights led by the Holy Roman Emperor. This led to the conflict known as the Investiture Controversy. Gregory was felt by many to have gone too far when he excommunicated the Holy Roman Emperor Henry IV and supported a rival claimant as emperor, and in 1080 the pro-imperial Synod of Brixen pronounced that Gregory was deposed and replaced as pope by Guibert.

Consecrated as Pope Clement III in Rome in March 1084, he commanded a significant following in Rome and elsewhere, especially during the first half of his pontificate, and reigned in opposition to four successive popes in the anti-imperial line: Gregory VII, Victor III, Urban II, and Paschal II. After his death and burial at Civita Castellana in 1100 he was celebrated locally as a miracle-working saint, but Paschal II and the anti-imperial party soon subjected him to damnatio memoriae, which included the exhuming and dumping of his remains in the Tiber. He is considered an anti-pope by the Roman Catholic Church.