Sergius III returned from exile to take over the papacy from the deposed antipope Christopher.

Pope Sergius III was the bishop of Rome and nominal ruler of the Papal States from 29 January 904 to his death. He was pope during a period of violence and disorder in central Italy, when warring aristocratic factions sought to use the material and military resources of the papacy. At the behest of Theophylact I of Tusculum, Sergius seized the papal throne from Antipope Christopher, who in turn had deposed Pope Leo V. Sergius' reign was subsequently marked by Theophylact's influence. As pope, Sergius continued many ecclesiastical controversies of his predecessors, including conflict over Pope Formosus' legacy, annulling all ordinations made by the late pope, and the filioque controversy with eastern patriarchs. His pontificate was similarly marked by temporal conflicts, with Sergius' refusal to crown Berengar I of Italy as Holy Roman Emperor, and his support of Byzantine Emperor Leo VI the Wise's fourth marriage. Sergius also saw the restoration of the Lateran Palace.

Christopher claimed the papacy from October 903 to January 904. Although he was listed as a legitimate pope in most modern lists of popes until the first half of the 20th century, the apparently uncanonical method by which he obtained the papacy led to his being removed from the quasi-official roster of popes, the Annuario Pontificio. As such, he is now considered an antipope by the Catholic Church.