(29 Sept. 855–17 Apr. 858)
A Roman, with a great reputation for piety and learning, he was educated in the Lateran school, ordained subdeacon by Gregory IV, and made cardinal priest of S. Callisto by Leo IV. On Leo's death (17 July 855) the first choice of clergy and people was Hadrian, cardinal priest of S. Marco, but on his refusal they elected Benedict. An influential imperialist group, however, preferred Anastasius, the ambitious cardinal priest whom Leo IV had anathematized and deposed but who had found protection with Emperor Louis II (855–75). Exploiting the fact that Benedict could not be consecrated save with the emperor's consent and in the presence of his envoys, they had his election disallowed, put forward Anastasius as pope, and brought him to Rome, where he was installed in the Lateran while Benedict, dragged from the papal throne, was ejected and on 21 Sept. imprisoned. Only when the general support for Benedict and the revulsion for his rival became clear did the envoys and the imperial party give way and allow Benedict's consecration to go ahead. This was a notable reverse for Louis, but his agents obliged Benedict to treat Anastasius and his adherents leniently and to accept the surveillance of Bishop Arsenius, Anastasius' uncle and leading partisan, in the role of imperial representative in Rome.
Only scattered glimpses survive of Benedict's brief reign, which seems in several respects to have foreshadowed that of his energetic successor Nicholas I, on whose counsel he in fact relied heavily. Emperor Lothair I (840–55) having died on the day of his consecration, he intervened, in ways which remain obscure, to ensure a peaceful settlement, at least temporarily, between his sons Lothair II (855–69), Louis II (855–75), and Charles the Bald (875–77). He did not hesitate to threaten Hubert, brother of Queen Theutberga of Lorraine, with excommunication for plundering monasteries, or to demand of Louis II that Ingeltrude, wife of Count Boso, who had fled with her lover to Lothair II's domains, should be fetched back to her husband. He took up vigorously, though unsuccessfully, the cause of four bishops in Brittany who had been uncanonically removed and replaced by the local prince; and although at the request of Hincmar, archbishop of Reims (845–82), he endorsed the controversial council of Soissons (853) which Leo IV had repudiated, he added the cautious proviso that his endorsement was subject to the report he had received being correct. Towards Constantinople he firmly asserted Rome's primatial jurisdiction, and when invited by Patriarch Ignatius to confirm his deposition of Gregory of Syracuse and other Sicilian bishops, he declined to do so until both parties had come to Rome and he had examined the case himself.
Aethelwulf, king of Wessex (839–58), with his son Alfred resided in Rome as a pilgrim during the first year of Benedict's reign, and not only completed the repair of the Saxon compound (near S. Spirito in Sassia) but made sumptuous gifts to the Roman churches and people (at the pope's behest) and promised annual contributions of money from England in future. Benedict restored the baptistery of Sta Maria Maggiore and several other churches, including St Paul's, and completely reconstructed the cemetery of S. Marco. His reign witnessed repeated floodings of the Tiber, and it fell to him to make good the extensive damage.