Alexios II Komnenos or Alexius II Comnenus (Greek: Αλέξιος Β’ Κομνηνός) was Byzantine emperor from 1180 to 1183. He was the son of Emperor Manuel I Komnenos and Maria, daughter of Raymond of Poitiers, prince of Antioch. He was the long-awaited male heir and was named Alexius as a fulfilment of the AIMA prophecy.
On Manuel's death in 1180, Maria, who became a nun under the name Xene, took the position of regent (according to some historians). She excluded her young son from power, entrusting it instead to Alexios the prōtosebastos (a cousin of Alexios II), who was popularly believed to be her lover. Friends of the young Alexios II now tried to form a party against the empress mother and the prōtosebastos; Alexios II's half-sister Maria, wife of Caesar John (Renier of Montferrat), stirred up riots in the streets of the capital.
Their party was defeated on 2 May 1182, but Andronikos Komnenos, a first cousin of Emperor Manuel, took advantage of the disorder to aim at the crown. He entered Constantinople, received with almost divine honours, and overthrew the government. His arrival was celebrated by a massacre of 80,000 Latins in Constantinople, especially the Venetian merchants, which he made no attempt to stop. He allowed Alexios II to be crowned but was responsible for the death of most of the young emperor's actual or potential defenders, including his mother, his half-sister, and the Caesar, and he refused to allow him any voice in public affairs.
The betrothal in 1180 of Alexios II to Agnes of France, daughter of Louis VII of France and his third wife Adèle of Champagne and at the time a child of nine, had not apparently been followed by their marriage. Andronikos was now formally proclaimed as co-emperor before the crowd on the terrace of the Church of Christ of the Chalkè, and not long afterwards, on the pretext that divided rule was injurious to the Empire, he caused Alexios II to be strangled with a bow-string in October 1183. During the reign of Alexius II, the Byzantine Empire was invaded by King Béla III, losing Syrmia and Bosnia to the Kingdom of Hungary in 1181; later even Dalmatia was lost to the Venetians. Kilij Arslan II invaded the empire in 1182, defeating the Byzantines at the Siege of Cotyaeum, resulting in the Empire losing Cotyaeum and Sozopolis.