(antipope 5 July 767–6 Aug. 768: d. ?)
A layman, brother of Duke Toto (Theodore) of Nepi, he irregularly succeeded Paul I, whose harsh rule and reliance on the ecclesiastical bureaucracy had incensed the lay aristocracy. For them it was important to have a pope they could influence, not least because the papacy had become a temporal power with the foundation of the papal state. As Paul lay dying, Toto plotted his murder, but, along with other Roman leaders, was persuaded by Christopher, the chief notary, that the subsequent election must follow traditional form. On Paul's death (28 June 767), however, Toto broke his oath, had Constantine acclaimed pope by a mob of his soldiers and dependants, installed him in the Lateran, and forced Bishop George of Praeneste first to ordain him subdeacon and deacon, and then, with two other bishops, to consecrate him in St Peter's (5 July). Constantine at once informed Pepin III, king of the Franks (751–68) and protector of the holy see, of his election, begging him to maintain the pact he had made with the two previous popes. Receiving no reply, he wrote again in Sept., using as a pretext an important letter which had arrived (12 Aug.) from Theodore, the new patriarch of Jerusalem; it was clear from his message that he was already encountering difficulties. In fact, the clerical party at Rome had regrouped and its leader, Christopher, who had fled to a monastery at Rieti with his son Sergius, had made contact with the duke of Spoleto and the Lombard king Desiderius (757–74). The Lombards were only too glad to exploit the situation, and with troops supplied by them and accompanied by Desiderius' envoy, the priest Waldipert, Christopher's son Sergius carried out a coup in Rome on 30 July 768. Toto was killed in street-fighting and Constantine fled to the Lateran oratory, where he was soon arrested. The Lombards momentarily sought to set up a pope of their own, the presbyter Philip, but he was almost at once ejected from the Lateran.
A new pope, Stephen III (IV), having been canonically elected, Constantine was dragged from his hiding-place, paraded ignominiously round the city, and at a synod on 6 Aug. stripped of the insignia of office and formally deposed. He was then imprisoned in a monastery where, attacked by a gang, he had his eyes gouged out. Finally, on 12 and 13 Apr. 769 he appeared before a synod held by Stephen III in the Lateran to settle the matter in proper form. At the first session he pleaded that the office of pope had been forced on him, but then abjectly admitted his guilt. At the second session he altered his tune, invoking precedents for laymen and even married men being made bishops. This exasperated his judges, who manhandled him and threw him out. The acts of his election (signed by all the clergy, including Stephen) and of his administration were burned, his ordinations were declared invalid, and he himself was sentenced to lifelong penance in a monastery. From this point he disappears from history.