Silvester III (or Sylvester), né John, was a pope. When Pope Benedict IX was driven from Rome in September 1044, John, bishop of Sabina, was elected after fierce and protracted infighting and took the title of Silvester III in January 1045. He was later charged, probably falsely, with having bribed his way into the election. Benedict issued an excommunication of the new pope and within three months returned to Rome and expelled his rival, who himself returned to Sabina to again take up his office of bishop in that diocese. Nearly three years later (December 1046) the council of Sutri deprived him of his bishopric and priesthood and ordered him sent to a monastery. This sentence was obviously suspended because he continued to function and was recognized as Bishop of Sabina until at least 1062. A successor bishop to the see of Sabina is recorded for October 1063, indicating that John must have died prior to this date.

While the right of Silvester III to be considered an authentic pope is open to some question, and some consider him to have been an anti-pope, he continues to be listed as an official pope in Vatican lists. A similar situation applies to both Pope Gregory VI and Pope Clement II.