The House of Komnenos (pl. Komnenoi; Greek: Κομνηνός, pl. Κομνηνοί, pronounced [komniˈni]), Latinized as Comnenus (pl. Comneni), was a Byzantine Greek noble family who ruled the Byzantine Empire in the 11th and 12th centuries. The first reigning member, Isaac I Komnenos, ruled from 1057 to 1059. The family returned to power under Alexios I Komnenos in 1081 who established their rule for the following 104 years until it ended with Andronikos I Komnenos in 1185. In the 13th century, they founded the Empire of Trebizond, a Byzantine rump state which they ruled from 1204 to 1461. At that time, they were commonly referred to as Grand Komnenoi (Μεγαλοκομνηνοί, Megalokomnenoi), a style that was officially adopted and used by George Komnenos and his successors. Through intermarriages with other noble families, notably the Doukas, Angelos, and Palaiologos, the Komnenos name appears among most of the major noble houses of the late Byzantine world.

The 11th-century Byzantine historian Michael Psellos reported that the Komnenos family originated from the village of Komne, near Philippopolis, in Thrace—usually identified with the "Fields of Komnene" (Κομνηνῆς λειμῶνας) mentioned in the 14th century by John Kantakouzenos—a view commonly accepted by modern scholarship. The first known member of the family, Manuel Erotikos Komnenos, acquired extensive estates at Kastamon in Paphlagonia, which became the stronghold of the family in the 11th century. The family thereby quickly became associated with the powerful and prestigious military aristocracy (dynatoi) of Asia Minor, so that despite coming from Thrace it came to be considered "eastern". Aside from deriving legitimacy as rulers from familial links to the prominent Doukai (emperors Constantine X and Michael VII in particular), they also had a tradition linking them to Claudius Gothicus, the supposed grandfather of Constantine the Great. Many classical monuments dedicated to Claudius stood in the vicinity of Kastra Komnenon, which according to historian Maximilian C. G. Lau may have increased his appeal in the eyes of the Komnenoi.

The 17th-century French scholar du Cange suggested that the family descended from a Roman noble family that followed Constantine the Great to Constantinople, from whose cousin but although such mythical genealogies were common—and are attested for the closely related Doukas clan as well—the complete absence of any such assertion in the Byzantine sources argues against Du Cange's view. The Romanian historian George Murnu suggested in 1924 that the Komnenoi were of Aromanian descent, but this view too is now rejected. Modern scholars consider the family to have been entirely of Greek origin.

Manuel Erotikos Komnenos was the father of Isaac I Komnenos (r. 1057–1059), and grandfather, through Isaac's younger brother John Komnenos, of Alexios I Komnenos (r. 1081–1118).

Isaac I Komnenos, a stratopedarch of the East under Michael VI, founded the Komnenos dynasty of Byzantine emperors. In 1057 Isaac led a coup against Michael and was proclaimed emperor. Although his reign lasted only until 1059, when his courtiers pressured him to abdicate and become a monk, Isaac initiated many useful reforms. The dynasty returned to the throne with the accession of Alexios I Komnenos, Isaac I's nephew, in 1081. By this time, descendants of all the previous dynasties of Byzantium seem to have disappeared from the realm, such as the important Scleros and Argyros families. Descendants of those emperors lived abroad, having married into the royal families of Georgia, Russia, France, Persia, Italy, Germany, Poland, Bulgaria, Hungary and Serbia; this made it easier for the Komnenos family to ascend to the throne.

Upon their rise to the throne, the Komnenoi became intermarried with the previous Doukas dynasty: Alexios I married Irene Doukaina, the grandniece of Constantine X Doukas, who had succeeded Isaac I in 1059. Thereafter the combined clan was often referred to as Komnenodoukai (Κομνηνοδούκαι) and several individuals used both surnames together. Several families descended from this wider clan, such as Palaiologos, Angelos, Vatatzes and Laskaris. Alexios and Irene's youngest daughter Theodora ensured the future success of the Angelos family by marrying into it: Theodora's grandsons became the emperors Isaac II Angelos (reigned 1185–1195 and 1203–1204) and Alexios III Angelos (reigned 1195–1203).

Under Alexios I and his successors the Empire was fairly prosperous and stable. Alexios moved the imperial palace to the Blachernae section of Constantinople. Much of Anatolia was recovered from the Seljuk Turks, who had captured it just prior to Alexios' reign. Alexios also saw the First Crusade pass through Byzantine territory, leading to the establishment of the Crusader states in the east. The Komnenos dynasty was very much involved in crusader affairs, and also intermarried with the reigning families of the Principality of Antioch and the Kingdom of Jerusalem; Theodora Komnene, niece of Manuel I Komnenos, married Baldwin III of Jerusalem, and Maria, grandniece of Manuel, married Amalric I of Jerusalem.

Remarkably, Alexios ruled for 37 years, and his son John II ruled for 25, after uncovering a conspiracy against him by his sister, the chronicler Anna Komnene. John's son Manuel ruled for another 37 years.