John II Komnenos or Comnenus (Greek: Ἱωάννης ὁ Κομνηνός, romanized: Iōánnēs Komnēnós; 13 September 1087 – 8 April 1143) was Byzantine emperor from 1118 to 1143. Also known as "John the Beautiful" or "John the Good" (Greek: Καλοϊωάννης, romanized: Kaloïōánnēs), he was the eldest son of Emperor Alexios I Komnenos and Irene Doukaina and the second emperor to rule during the Komnenian restoration of the Byzantine Empire. As he was born to a reigning emperor, he had the status of a porphyrogennetos. John was a pious and dedicated monarch who was determined to undo the damage his empire had suffered following the Battle of Manzikert, half a century earlier.

John has been assessed as the greatest of the Komnenian emperors. This view became entrenched due to its espousal by George Ostrogorsky in his influential book History of the Byzantine State, where John is described as a ruler who, "... combined clever prudence with purposeful energy ... and [was] high principled beyond his day." In the course of the quarter-century of his reign, John made alliances with the Holy Roman Empire in the west, decisively defeated the Pechenegs, Hungarians and Serbs in the Balkans, and personally led numerous campaigns against the Turks in Asia Minor. John's campaigns fundamentally changed the balance of power in the east, forcing the Turks onto the defensive; they also led to the recapture of many towns, fortresses and cities across the Anatolian peninsula. In the southeast, John extended 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 marched into Muslim Syria at the head of the combined forces of Byzantium and the Crusader states; yet despite the great vigour with which he pressed the campaign, John's hopes were disappointed by the evasiveness of his Crusader allies and their reluctance to fight alongside his forces.

Under John, the empire's population recovered to about 10 million people. The quarter-century of John II's reign is less well recorded by contemporary or near-contemporary writers than the reigns 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.

The Latin historian William of Tyre described John as short and unusually ugly, with eyes, hair and complexion so dark he was known as 'the Moor'. Yet despite his physical appearance, John was known as Kaloïōannēs, "John the Good" or "John the Beautiful"; the epithet referred to his character. Both his parents were unusually pious and John surpassed them. Members of his court were expected to restrict their conversation to serious subjects only. The food served at the emperor's table was very frugal and John lectured courtiers who lived in excessive luxury. His speech was dignified, but he engaged in repartee on occasion. All accounts agree that he was a faithful husband to his wife, an unusual trait in a medieval ruler. Despite his personal austerity, John had a high conception of the imperial role and would appear in full ceremonial splendour when this was advantageous.

John was famed for his piety and his remarkably mild and just reign. He is considered an exceptional example of a moral ruler, at a time when cruelty was the norm. He is reputed never to have condemned anyone to death or mutilation. Charity was dispensed lavishly. For this reason, he has been called the Byzantine Marcus Aurelius. By the example of his personal morality and piety he effected a notable improvement in the manners of his age. Descriptions of him and his actions indicate that he had great self-control and personal courage, and was an excellent strategist and general.

John was born on 13 September 1087, the third child and first son of Emperor Alexios I Komnenos and his wife Irene Doukaina. Joannes Zonaras describes John's birth, "And then a son was born to the emperor, who was honoured with the holy baptism in the holy church of Hagia Sophia by the Patriarch. He was named John and his father then crowned him with a diadem". At a young age he was associated with his father as co-emperor, sometime between 1 September and early November, 1092. The first act in the name of both emperors was issued on 15 November 1092. The occasion of John's elevation was celebrated by a special issue of coins with his parents depicted on the reverse and John crowned by Christ on the obverse.

Alexios I arranged a dynastic marriage for John. In 1104 or early 1105 John was married to the Hungarian princess Piroska, daughter of King Ladislaus I of Hungary. She was given the Greek name Irene, meaning 'peace', in Byzantium. This marriage sealed a political alliance, which would strengthen Byzantium's position in the Balkans and Adriatic. In 1106 Alexios I was campaigning against the Serbs, most probably accompanied by his son John. At this time John's wife was heavily pregnant, she was with her husband on campaign, giving birth to twins, Alexios and Maria, in Balabista, Macedonia. The question arises as to why the birth was not in Constantinople. It is reasonably supposed that John did not wish to leave his unsupported wife under the control of his mother, who would have had authority in Constantinople in the absence of both emperors. In 1108, the Treaty of Devol recorded the submission of the defeated Bohemond I of Antioch to " ... the great Emperor Kyr[ios] Alexios Komnenos and of your thrice-loved son, Emperor Kyr John born in the purple". This leaves no doubt as to John's status as co-emperor and heir apparent.

John II succeeded his father as ruling basileus in 1118. Despite his earlier coronation as co-emperor, the accession of John was contested. That Alexios I favoured John to succeed him is made obvious by the elevation of his son to the position of co-emperor. However, Alexios' influential wife, Irene, favoured the Caesar Nikephoros Bryennios, the husband of her eldest child Anna Komnene. Anna, who in infancy had been betrothed to her father's first co-emperor Constantine Doukas, herself harboured obvious aspirations to power and the throne. During Alexios' final illness both wife and daughter exploited his physical weakness to apply pressure on him in support of their agenda for the succession. Alexios endured these constant demands without formally changing his intended successor. As Alexios lay dying in the monastery of the Mangana on 15 August 1118, John, relying on trusted relatives, especially his brother Isaac Komnenos, gained entry into the monastery and obtained the imperial signet ring from his father. He then assembled his armed followers and rode to the Great Palace, gathering the support of the citizenry on the way. The palace guard at first refused to admit John without clear proof of his father's wishes, however, the mob surrounding the new emperor simply forced an entry. In the palace John was acclaimed emperor. Irene, taken by surprise, was unable either to persuade her son to step down, or to induce Nikephoros to contend for the throne. In a recent biography of Anna, however, this account of events has been disputed, in particular the involvement of John's sister in any palace coup attempt during the days around Alexios' death, has been questioned. The weight of historical opinion remains that Anna tried at least twice to usurp her brother's throne, resulting in her eventual perennial 'house arrest'. Anna later complained that John and his successor, her nephew Emperor Manuel, kept her isolated from society for 30 years.