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For several other persons named Constantine Doukas, see Constantine Doukas (disambiguation).

For the Moldavian ruler of the same name, see Constantine Ducas.

Constantine Doukas or Ducas (Greek: Κωνσταντίνος Δούκας, Kōnstantinos Doukas), (c. 1074–c. 1095) was Byzantine co-emperor from c. 1075 to 1078 and from 1081 to 1087. He was the son of Emperor Michael VII Doukas and his Georgian wife Maria of Alania.

Constantine was porphyrogennētos, i.e., born in the purple, and was associated on the throne by his father soon after his birth. He was quickly involved in imperial diplomacy, as the infant was betrothed to Olympias of Apulia, renamed Helena, the daughter of the Norman leader Robert Guiscard and Sikelgaita. After his father was forced to abdicated in 1078, Constantine's mother Maria married the new Emperor Nikephoros III Botaneiates, but failed in convincing him to recognize the imperial status and succession rights of her son. Constantine's demotion also involved the breaking off of the engagement to the daughter of Robert Guiscard, who used this as pretext to commence military action against the Byzantine Empire.

After the fall of Botaneites in 1081, Maria's intimacy (of whatever nature) with his successor Alexios I Komnenos and the latter's policy of alliance with the Doukas family brought about Constantine's restoration as co-emperor. His signature was appended to all official documents issued by Alexios, and he accompanied the emperor on public occasions. When Anna Komnene, the first child of Alexios I and Irene Doukaina was born, she was promptly betrothed to Constantine, which confirmed his positions as heir. Maria's request, a noted philosopher Theophylaktos of Ohrid wrote a special treatise for the young prince’s education.

However, Constantine's position was undermined by his weak constitution and, more immediately, by the birth of a son (the future John II Komnenos) to Alexios I in 1087. Although now deprived of his status as co-emperor and imperial heir, Constantine retained the emperor's favor and remained on good terms with him. Maria was forced to retire to a monastery. During Alexios' campaign against the Serbians in 1094 Constantine dined and entertained the emperor at his own expense. Later he refused to participate in a rebellion against Alexios. Constantine apparently died in c. 1095.

His daughter Theodora married the future doge of Venice Domenico Selvo.

• Polemis, D.I. (1968). The Doukai. 
• (1966) in J. M. Hussey: The Cambridge Medieval History, Volume IV The Byzantine Empire, Part 1 Byzantium and Its Neighbours. London: The Syndics of the Cambridge University Press.