Pope John X (Latin: Ioannes X; died 28 May 928) was the bishop of Rome and nominal ruler of the Papal States from March 914 to his death. A candidate of the counts of Tusculum, he attempted to unify Italy under the leadership of Berengar of Friuli, and was instrumental in the defeat of the Saracens at the Battle of Garigliano. He eventually fell out with Marozia, who had him deposed, imprisoned, and finally murdered. John’s pontificate occurred during the period known as the Saeculum obscurum.

John X, whose father’s name was also John, was born at Tossignano, along the river Santerno. He was made a deacon by Peter IV, the bishop of Bologna, where he attracted the attention of Theodora, the wife of Theophylact I of Tusculum, the most powerful noble in Rome. John was a relative of Theodora's family. Liutprand of Cremona alleged that John became her lover during a visit to Rome; However, Johann Peter Kirsch says, "This statement is, however, generally and rightly rejected as a calumny. Liutprand wrote his history some fifty years later, and constantly slandered the Romans, whom he hated. At the time of John's election Theodora was advanced in years, and is lauded by other writers (e.g. Vulgarius)."

It was through Theodora’s influence that John was on the verge of succeeding Peter as bishop of Bologna, when the Archbishopric of Ravenna became available. He was consecrated as archbishop in 905 by Pope Sergius III, another clerical candidate of the counts of Tusculum. During his eight years as archbishop, John worked hard with Sergius in an unsuccessful attempt to depose Louis the Blind and have Berengar of Friuli, who claimed the Kingdom of Italy, crowned emperor in his stead. He also had to defend himself from a usurper who tried to take his episcopal see away, as well as confirming his authority over Nonantola Abbey when the abbot attempted to free it from the jurisdiction of the archbishop of Ravenna.

After the death of Pope Lando in 914, a faction of the Roman nobility, headed by Theophylact of Tusculum, summoned John to Rome to assume the vacant papal chair. Although this was again interpreted by Liutprand as Theodora personally intervening to have her lover made pope, it is far more likely that John’s close working relationship with Theophylact, and his opposition to the ordinations of Pope Formosus, were the real reasons for his being transferred from Ravenna to Rome. Since switching sees was considered an infraction of canon law, as well as contravening the decrees of the Lateran Council of 769, which prohibited the installation of a pope without election, John’s appointment was criticised by his contemporaries. Nevertheless, while Theophylact was alive, John adhered to his patron’s cause.

The first task that confronted John X was the existence of a Saracen outpost on the Garigliano River, which was used as a base to pillage the Italian countryside. John consulted Landulf I of Benevento, who advised him to seek help from the Byzantine Empire and Alberic I of Spoleto. John took his advice and sent papal legates to Berengar, various Italian princes, as well as to Constantinople, seeking help to throw out the Saracens. The result was a Christian alliance, a precursor to the Crusades of the following century. The forces of the new Byzantine strategos of Bari, Nicholas Picingli, joined those of various other south Italian princes: Landulf I of Benevento, John I and Docibilis II of Gaeta, Gregory IV and John II of Naples, and Guaimar II of Salerno. Meanwhile, Berengar brought with him troops from the northern parts of Italy, and the campaign was coordinated by John X, who took to the field in person, alongside Duke Alberic I of Spoleto.

After some preliminary engagements at Campo Baccano and at Trevi, the Saracens were driven to their stronghold on the Garigliano. There, at the Battle of Garigliano, the allies proceeded to lay siege to them for three months, at the end of which the Saracens burnt their houses and attempted to burst out of the encirclement. With John leading the way, all were eventually caught and killed, achieving a great victory and removing the ongoing Saracen threat on the Italian mainland. John then confirmed the granting of Traetto to the Duke of Gaeta, as a reward for abandoning his Saracen allies.

Berengar had pressed for the imperial crown ever since he had defeated and driven Emperor Louis the Blind out of Italy in 905. John X used this as leverage to push Berengar into supporting and providing troops to the Saracen campaign. Having completed his end of the bargain, Berengar now insisted that John do likewise. So in December 915, Berengar approached Rome, and after being greeted by the family of Theophylact (whose support he secured), he met Pope John at St. Peter’s Basilica. On Sunday 3 December, John crowned Berengar as emperor, while Berengar in turn confirmed previous donations made to the See of Peter by earlier emperors.

Although Berengar had the support of the major Roman nobility and the pope, he had enemies elsewhere. In 923, a combination of the Italian princes brought about the defeat of Berengar, again frustrating the hopes of a united Italy, followed by his assassination in 924. Then in 925 Theophylact of Tusculum and Alberic I of Spoleto also died; this meant that within the course of a year, three of Pope John’s key supporters had died, leaving John dangerously exposed to the ambitions of Theophylact’s daughter, Marozia, who, it was said, resented John’s alleged affair with her mother, Theodora.