Guibert or Wibert of Ravenna (c. 1029-8 January, 1100) was Antipope from 25 February, 1080 to his death. He was born in Parma and had family connections to the Counts of Canossa.

He served as Imperial Chancellor for Italy until 1063. Emperor Henry IV named Guibert, then Bishop of Ravenna, as pope over the opposition of Pope Alexander II, who, however accepted the situation once Guibert had been compelled to take an oath of allegiance. Shortly after Hildebrand was elected Pope Gregory VII Guibert became one of the most visible leaders of opposition to the Gregorian reforms.

Having attended one of Gregory's Lenten Synods, Guibert refused to attend the next, despite his promise to do so. A synod of bishops then deposed Gregory, and a sequence of excommunications and depositions followed. In a June 1080 synod summoned by Henry Gregory was deposed again and Guibert elected Pope in his place. Henry then marched on Rome, Gregory fled, and Guibert was again elected Pope, as Clement III. The new pope then crowned Henry as Emperor in St Peter's Basilica.

Gregory returned with the army of his Norman ally Robert Guiscard. Henry and Clement were expelled from Rome, and Clement became archbishop of Ravenna. Clement returned to Rome functioning as Pope and negotiating with other European rulers during the reigns of Victor III and Urban II. At various times he had the backing of several European rulers, but there was an increase in support for Urban II. When Pope Paschal II was elected Clement prepared to press his claim but was forced to withdraw, dying in Civita' Castellana 8 January 1100.

The elevation of Guibert has to be seen in the wider context of the time: there had been several antipopes in the recent past there were political struggles within the empire, and the Investiture Crisis.

Clement legislated against simony and other practices, and, through the leeway he granted the cardinals supporting him, contributed to the develoment of the College of Cardinals.

See also: other popes named Clement.