Isaac Komnenos or Comnenus (Greek: Ἰσαάκιος Κομνηνός, c. 1050 – 1102/1104) was a notable Byzantine general in the 1070s and one of the major supporters of Byzantine Emperor Alexios I Komnenos (r. 1081–1118), who was his younger brother. Alexios created the title of sebastokrator for Isaac.
Isaac was the second-eldest son and third child of the Domestic of the Schools John Komnenos and Anna Dalassene. As such he belonged to the highest aristocracy of mid-11th century Byzantium, being the nephew of Emperor Isaac I Komnenos (r. 1057–1059). In 1071 or 1072, the Emperor Michael VII Doukas (r. 1071–1078) married him to Irene, a Georgian princess and cousin to Michael's empress, Maria of Alania.
In the 1070s, following the disastrous Battle of Manzikert, Isaac was employed as military commander in Anatolia against the Seljuk Turks. In 1073, as Domestic of the Schools of the East (i.e. commander-in-chief of the eastern field army), he was captured by the Turks, and was released only after ransom was paid. In the next year, he was again sent East as doux of Antioch, he quelled local unrest, but was again captured by the Turks and had to be ransomed by the citizens of the city.
In the reign of Nikephoros III Botaneiates (r. 1078–1081) he enjoyed the emperor's favour, and plotted at court, using his influence with Empress Maria to advance the position of the Komnenos clan; especially that of his younger brother Alexios, who was directly related to the ruling Doukid dynasty through his marriage to Irene Doukaina. When the Komnenids finally made their move and rebelled against Nikephoros III, there was contention that Isaac could be proclaimed Emperor instead of Alexios, until the Doukids and George Palaiologos conspired for Alexius to be chosen. The Doukids continued to work against Isaac, until Isaac proved his loyalty by placing the emperor's purple boots on him. Thereafter he proved one of his most loyal, steadfast and enthusiastic supporters. Alexios in turn rewarded him by awarding him with the new title of sebastokrator which marked him as a near-equal; in the words of Anna Komnene, an "emperor without the purple".
He was physically similar to his brother Alexius, though paler and his beard less bushy. He reportedly enjoyed hunting and war, putting himself in the vanguard of battles.
Isaac Komnenos was married to the Georgian princess Irene, the cousin of the empress Maria of Alania, by whom he had several children:
- John Komnenos, protosebastos and governor (doux) of Dyrrhachium from 1092 to 1105/6
- Alexios Komnenos, pansebastos and doux of Dyrrhachium after 1105/6
- Constantine Komnenos, pansebastos and doux of Beroea, later megas droungarios
- Adrianos Komnenos, became a monk, later Archbishop of Bulgaria as John IV
- Sophia Komnene, married Nikephoros Dokeianos
- Eudokia Komnene, married Nikephoros Botaneiates
- Two unnamed daughters, one of them was engaged to Gregory Gabras