Antipope Constantine II (died 769?) was an antipope for over a year, from 28 June 767 to 6 August 768. He was overthrown through the intervention of the Lombards and tortured before he was condemned and expelled from the Church during the Lateran Council of 769.
Upon the death of Pope Paul I various factions contended to secure the appointment of their respective candidates as pope. Constantine, although a layman, was supported by a group of Tuscan nobles, led by his brother. They secured his election by force of arms. The following spring, local authorities, with Lombard support, succeeded in deposing him. The Lombards then attempted to install their own candidate, a priest named Philip. He, in turn, was overthrown the following day by the local authorities who then elected the churchman Stephen. For a short time Constantine retained some support outside the city, which resulted in armed conflict. The supporters of Stephen had the imprisoned Constantine blinded, which, it seems to be generally allowed, Stephen was unable to hinder. After which Constantine was held in close confinement in a monastery.
Constantine was born into a noble Roman family in Nepi near Viterbo. He was one of four brothers, of which the most prominent was Toto of Nepi. Toto, the papal governor and self-styled "Duke" of Nepi, began to position himself to take advantage of the expected death of Pope Paul I, and elevate his own candidate onto the papal throne. Christophorus, the Primicerius of the notaries, forced Toto to take an oath to respect the traditional clerical method of papal elections. Toto, however, having retired to his estates in Nepi, with the help of Constantine and his other brothers collected troops from his duchy and other parts of Tuscany, in addition to arming a group of peasants to swell the numbers.
On 28 June, hearing that Pope Paul was on the verge of dying, Toto and his armed men forced their way into the city through the Gate of St. Pancratius. With Paul’s death, Toto made his way to the Basilica of the Apostles where the other members of the papal court and Roman nobility were gathering, and there Christophorus had everyone swear that they would all uphold each other’s rights during the upcoming election. However, as soon as the meeting had broken up, Toto’s armed retainers had assembled in his house at Rome and elected his brother Constantine as pope.