John II Komnenos (or Latinised as Comnenus) (September 13, 1087 – April 8, 1143) is Byzantine Emperor from 1118 to 1143.

Also known as Kaloïōannēs ("John the Good" or "John the Beautiful"), he is the eldest son of Emperor Alexios I Komnenos and Irene Doukaina.

The second emperor of the Komnenian restoration of the Byzantine Empire.

John is a pious and dedicated monarch who is determined to undo the damage his empire had suffered following the battle of Manzikert, half a century earlier.

John has been assessed as having been the greatest of the Komnenian emperors.

In the course of his twenty-five year reign, John makes alliances with the Holy Roman Empire in the west, decisively defeats the Pechenegs, Hungarians and Serbs in the Balkans, and personally leads numerous campaigns against the Turks in Asia Minor.

John's campaigns fundamentally change the balance of power in the east, forcing the Turks onto the defensive and restoring to the Byzantines many towns, fortresses and cities right across the peninsula.

In the southeast, John extends Byzantine control from the Maeander in the west all the way to Cilicia and Tarsus in the east.

In an effort to demonstrate the Byzantine ideal of the emperor's role as the leader of the Christian world, John marches into Muslim Syria at the head of the combined forces of Byzantium and the Crusader states; yet despite the great vigor with which he presses the campaign, John's hopes are disappointed by the treachery of his Crusader allies, who deliberately fail to fight against their common Muslim enemies at the crucial moment.

Also under John, the empire's population recovers to about 10 million people.

Unfortunately, John's reign is less well covered by contemporary sources than those of either his father, Alexios I, or his son, Manuel I.

In particular little is known of the history of John's domestic rule or policies.