Isaac Komnenos or Comnenus (Greek: Ισαάκιος Κομνηνός, Isaakios Komnēnos) (after 16 January 1093 – after 1152) was the third son of Emperor Alexios I Komnenos and Irene Doukaina.

He was the brother of Emperor John II Komnenos and the historian Anna Komnene. Isaac was given the dignity of kaisar (Caesar) by his father.

After the succession of John II in 1118, Isaac was received at the imperial court almost as an equal to the emperor and received the elevated dignity of sebastokratōr. He engaged in charitable works, including the restoration of a monastery near the Blachernae palace. By 1130 John and Isaac had become estranged, and Isaac was forced to flee Constantinople for 6 years after an alleged conspiracy. Isaac sought refuge at several eastern courts, including those of the Danishmendid amir Gümüshtigin Ghazi II ibn Danishmend at Melitene, and in Jerusalem. In 1136 Isaac returned to Constantinople and was reconciled with his brother. At John II's death in 1143, Isaac was sent away to Herakleia Pontica, ostensibly to keep him from seizing power, which he was indeed tempted to do in 1145–1146. He may have been forced into a rural retirement and in 1152 endowed his own monastery near Ainos in Thrace.

This potential for conflict within the top tier of the court aristocracy would continue into the reign of Manuel I Komnenos, and eventually Isaac's son Andronikos became emperor in 1183.

Isaac Komnenos was married to Eirene, possibly a Kievan princess. He may also have married Kata of Georgia, daughter of David IV of Georgia and Rusudan of Armenia. By his first marriage, he had: # John Comnenus called Tzelepes (i.e., Çelebi), who became a Muslim. # Andronikos I Komnenos, emperor 1183–1185. # Maria Komnene, who married before 1166 Joseph Bryennios. # Anna Komnene, who married John Arbantenos. # Eudokia Komnene, who probably married Constantine Palaiologos. # Helena Komnene (d. 1183), who probably married Yury Dolgoruky of Kiev. He also had an illegitimate son, Alexios Komnenos.

* Paul Magdalino, The Empire of Manuel I Komnenos 1143–1180, Cambridge University Press, 1993. * K. Varzos, Ē genealogia tōn Komnēnōn, Thessalonikē, 1984.

* Genealogie Mittelalter