John Doukas or Ducas (Greek: Ιωάννης Δούκας, Iōannēs Doukas), (c. 1126 – c. 1200) was the eldest son of Constantine Angelos by Theodora Komnene, the seventh child of the Byzantine Emperor Alexios I Komnenos and Irene Doukaina, from whose family name John Doukas took his own.
Doukas is first attested in an imperial document in 1166. In 1176 he participated at the Battle of Myriokephalon as a military commander. The day after the battle he led an attack against the Seljuk Turks but they did not stand and fight and he failed to enforce the renewal of a general engagement. In 1185 he supported his nephew Isaac II Angelos in overthrowing Andronikos I Komnenos and seizing the imperial throne. Isaac II rewarded him with the high court dignity of sebastokratōr. For a while John Doukas was now the second most prominent man in the Empire, and in 1186 he led one of the exeditions against Peter and Asen, who had rebelled, trying to re-establish the independence of Bulgaria. In spite of his advanced age, John acquitted himself with some success. The suspicious emperor deprived him of his command, and in 1195 the elderly John supported the usurpation of his other nephew, Isaac's brother Alexios III Angelos.
His first marriage (his wife's name is unknown) produced two sons:
- Isaac Angelos, who married the daughter of Alexios Branas
- Alexios Doukas Komnenos Angelos
- Theodore Komnenos Doukas, who succeeded his half-brother Michael as ruler of Epirus
- Manuel Komnenos Doukas, who succeeded Theodore as ruler of Thessalonica
- Constantine Komnenos Doukas, who succeeded Theodore as ruler of Acarnania
He had three daughters, none of whose names is known for certain.
- A daughter, who married Mateo Orsini, count of Cephalonia; their son Riccardo was count of Cephalonia from 1260 to 1304.
- Another daughter married Michael Kantakouzenos, who, with Theodore Branas, John Petraliphas and others, conspired to overthrow Isaac II and to bring Alexios III Angelos to power in 1195.
He also had an illegitimate son by an unnamed mistress:
- Choniates, p. 106.
- Choniates, Niketas. Historia. English translation: Magoulias, H. (O City of Byzantium: Annals of Niketas Choniates). Detroit, 1984. ISBN 0-8143-1764-2
- D.I. Polemis, The Doukai, London, 1968.
- K. Varzos, Ē genealogia tōn Komnēnōn (Thessalonica, 1984) vol. 1 pp. 641–9; vol. 2 pp. 540–689.