Pope John XVIII (Latin: Ioannes XVIII; died June or July 1009) was Pope and ruler of the Papal states from January 1004 (25 December 1003 NS) to his abdication in June 1009. He was born Giovanni Fassano at Rome, the son of a Roman priest.
Pope John owed his election to the influence and power of the Crescentii clan. During his whole pontificate he was allegedly subordinate to the head of the Crescentii, who controlled Rome, the patricius (an aristocratic military leader) John Crescentius III.
This period was disrupted by continuing conflicts between the Ottonian Emperor Henry II and Arduin of Ivrea, who had styled himself King of Italy in 1002 after the death of Emperor Otto III. Rome was wracked with bouts of plague, and Saracens operated freely out of the Emirate of Sicily ravaging the Tyrrhenian coasts.
As Pope, John XVIII occupied his time mainly with details of ecclesiastical administration. He authorized a new Diocese of Bamberg to serve as a base for missionary activity among the Slavs, a concern of Henry II. He also adjudicated the over-reaching of the bishops of Sens and Orléans regarding the privileges of the abbot of Fleury. John was successful in creating, at least temporarily, a rapprochement between the Eastern and Western churches. His name could be found on Eastern diptychs and he was prayed for in masses in Constantinople.
Ultimately he abdicated and, according to one catalogue of Popes, retired to a monastery, where he died shortly afterwards. His successor was Pope Sergius IV.
Pope John XVIII was only the 17th pope called John, because John XVI (997–998) was an antipope according to official reckoning. His status as an antipope was not recognized at the time, however, so the true 16th Pope John called himself John XVII. The true 17th pope called John took the regnal number XVIII. The true sequence of numbering has never been corrected.