John XII (born 937? died May 14, 964), Pope from 955 to 964, was the son of Alberic II, whom he succeeded as patrician of Rome in 954, being then only eighteen years of age.

His original name was Octavian, but when he assumed the papal tiara as successor to Agapetus II, he adopted the apostolic name of John. This was the second example of the custom of altering the surname in connection with elevation to the papal chair, the first being Pope John II. As a temporal ruler John was devoid of the vigour and firmness of his father, and his union of the papal office, which through his scandalous private life he made a byword of reproach with his civil dignities proved a source of weakness rather than of strength. An account of the charges leveled against him from Patrologia Latina includes:

"Then, rising up, the cardinal priest Peter testified that he himself had seen [John XII] celebrate mass without taking communion. John, bishop of Narni, and John, a cardinal deacon, professed that they themselves saw that a deacon had been ordained in a horse stable, but were unsure of the time. Benedict, cardinal deacon, with other co-deacons and priests said they knew that he had been paid for ordaining bishops, specifically that he had ordained a ten-year-old bishop in the city of Todi... They testified about his adultery, which they did not see with their own eyes, but nonetheless knew with certainty: he had fornicated with the widow of Rainier, with Stephana his father's concubine, with the widow Anna, and with his own niece, and he made the sacred palace into a whorehouse. They said that he had gone hunting publicly; that he had blinded his confessor Benedict, and thereafter Benedict had died; that he had killed John, cardinal subdeacon, after castrating him; and that he had set fires, girded on a sword, and put on a helmet and cuirass. All, clerics as well as laymen, declared that he had toasted to the devil with wine. They said when playing at dice, he invoked Jupiter, Venus and other demons. They even said he did not celebrate Matins and the canonical hours nor did he make the sign of the cross."

In order to protect himself against the intrigues in Rome and the power of Berengar II of Italy , he called to his aid Otto the Great of Germany, to whom he granted the imperial crown on February 2, 962.

Even before Otto left Rome the pope had, however, repented of his recognition of a power which threatened altogether to overshadow his authority, and had begun to conspire against the new emperor. His intrigues were discovered by Otto, who, after he had defeated and taken prisoner Berengar, returned to Rome and summoned a council which deposed John, who was in hiding in the mountains of Campania, and elected Leo VIII in his stead.

An attempt at an insurrection was made by the inhabitants of Rome even before Otto left the city, and on his departure John returned at the head of a formidable company of friends and retainers, and caused Leo to seek safety in immediate flight. Otto determined to make an effort in support of Leo, but before he reached the city John had died. The manner of his death is uncertain, although it was rumored that John was murdered by a jealous husband whose wife had been discovered receiving the sexual affections of the Pope.

Benedict V soon succeeded him.