Guibert or Wibert of Ravenna (c. 1029 – 8 September 1100) is an Italian prelate, archbishop of Ravenna, who is elected pope in 1080 in opposition to Pope Gregory VII.

Gregory is the leader of the movement in the church which opposes the traditional claim of European monarchs to control ecclesiastical appointments, and this is opposed by supporters of monarchical rights led by the Holy Roman Emperor.

This leads to the conflict known as the Investiture Controversy.

Gregory is felt by many to have gone too far when he excommunicated the Holy Roman Emperor Henry IV and support a rival claimant as emperor, and in 1080 the pro-imperial Synod of Brixen pronounces that Gregory is deposed and replaced as pope by Guibert.

Consecrated as Pope Clement III in Rome in March of 1084, he commands a significant following in Rome and elsewhere, especially during the first half of his pontificate, and reigns 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 is celebrated locally as a miracle-working saint, but Paschal II and the anti-imperial party soon subject him to a thorough deletio and damnatio memoriae, which includes the exhuming and dumping of his remains in the Tiber.

He is now considered an anti-pope by the Roman Catholic Church.