John II Comnenus (September 13, 1087 - April 8, 1143) was Byzantine emperor from 1118 to 1143. Also known as Kalo Ioannes (John the Beautiful), he was the eldest son of the emperor Alexius, whom he succeeded in 1118.

On account of his mild and just reign he has been called the Byzantine Marcus Aurelius. By the personal purity and piety of his character he effected a notable improvement in the manners of his age, and he devoted his reign to restoring the Byzantine Empire to its former extents, before the disaster at Manzikert in 1071. His various successes against the invading Petchenegs, Serbians and Seljuk Turks, whose progress in Asia Minor he reverted, along with his attempts to establish Byzantine suzerainity over the Crusader States in Antioch and Edessa, did much to restore the reputation of his empire, even if the Comnenian dynasty in hindsight gave Byzantium, torn apart by corruption and invasions, but a brief respite against disintegration. His only serious - but very telling - setbacks were against the Venetians, upon whose naval strength the empire was dependent after the breakdown of the Byzantine fleet in the 11th century. An effort to reduce their extensive privileges within the empire ended with a humiliating return to status quo, many Byzantine ports being plundered in the process.

John married Piroska , daughter of King Ladislaus I of Hungary, in 1104. Their two eldest sons predeceased him, and he passed over the third son, Isaakios, in favor of the youngest, Manuel, who had shown great promise. After his death Piroska was venerated as St. Eirene .

He was accidentally poisoned by his own arrow during a wild-boar hunt on Mount Taurus, on April 8, 1143.