|Emperor of the Romans|
|Successor||Nikephoros III Botaneiates|
|Co-emperors||Michael VII Doukas (1071–1078)|
Konstantios Doukas (1071–1078)
Andronikos Doukas (1068–1070s)
|Predecessor||Nikephoros III Botaneiates|
|Successor||Alexios I Komnenos|
|Mother||Maria of Alania|
Constantine Doukas or Ducas (Greek: Κωνσταντίνος Δούκας, Kōnstantinos Doukas), (late 1074 – c. 1095) was Byzantine junior emperor from 1074 to 1078, and again from 1081 to 1087. He was born to Emperor Michael VII and Empress Maria of Alania in late 1074, and elevated to junior emperor in the same year. He was junior emperor until 1078, when Michael VII was replaced by Nikephoros III Botaneiates. Because Constantine was not made junior emperor under Nikephoros III, his betrothal to Olympias, the daughter of Robert Guiscard, was broken, which Robert Guiscard used as a pretext to invade the Byzantine Empire. John Doukas forced Nikephoros to abdicate to Alexios I Komnenos in 1081, and shortly after Alexios elevated Constantine to junior emperor under himself. Constantine remained junior emperor until 1087, when Alexios had a son, John II Komnenos. Constantine died in c. 1095.
Constantine Doukas was born in late 1074 to Byzantine Emperor Michael VII and his wife Maria of Alania, as a porphyrogennetos, meaning he was born during his father's reign. Constantine was elevated to junior emperor in 1074 by Michael VII. Shortly after his birth, in August 1074, Constantine was betrothed to Olympias, the daughter of Robert Guiscard, the Norman Duke of Sicily. This arrangement was cancelled after Michael abdicated in 1078, whereupon Maria and Constantine retired to the Monastery of Petrion. Maria married Nikephoros III Botaneiates, who seized power after Michael's abdication, at the urging of Michael's uncle John Doukas, but was unable to convince him to elevate Constantine to junior emperor, thereby breaking the betrothal. Robert Guiscard therefore launched an invasion of the Byzantine Empire, using the broken betrothal as a pretext.
In order to combat this invasion, Alexios I Komnenos was given a large force to repel the Norman army led by Guiscard. John Doukas, who had previously urged Nikephoros to seize power, conspired against Nikephoros, intending to overthrow him and replace him with Alexios. Nikephoros, unable to form an alliance with either the Seljuks or Nikephoros Melissenos, was forced to abdicate to Alexios in 1081. After Alexios ascended the throne in 1081, he elevated Constantine to junior emperor, and betrothed his daughter Anna Komnene to him in 1083, shortly after her birth. However, he was replaced as junior emperor and imperial heir by John II Komnenos, in 1087, shortly after his birth to Alexios and Irene Doukaina. Constantine died in c. 1095.
Constantine Doukas is thought to be engraved on the Holy Crown of Hungary, which was given to Hungarian King Géza I of Hungary by Constantine's father Michael VII, depicted alongside King Geza I and Michael VII; although the figure may actually be Konstantios Doukas.