John Doukas (or Ducas) (, Iōannēs Doukas) (died c. 1088) was the son of Andronikos Doukas, a Paphlagonian Greek nobleman who may have served as governor of the theme of Bulgaria (Moesia), and the younger brother of Emperor Constantine X Doukas. John Doukas was the paternal grandfather of Irene Doukaina, wife of Emperor Alexios I Komnenos. Career as Caesar John Doukas, who was given the court dignity of Caesar by his brother Constantine X, was one of the most influential members of the court aristocracy from the death of his brother into that of Alexios I Komnenos. His wealth derived of estates in Thrace and Bithynia, and he was a close friend of the historian Michael Psellos. Although he is usually documented by the sources as a member of the court, he had begun his career as a general. After serving as a counsellor and supporter of his brother, John came to the fore after his brother's death in 1067 as the natural protector of the rights of his nephew Michael VII Doukas. His position as Caesar and his family's influence in the Senate meant that he was behind the opposition of the court officials to the Empress Mother Eudokia Makrembolitissa and her marriage to Romanos IV Diogenes. Over the course of the next three years he became the emperor's bitterest enemy, but his intriguing meant that the Caesar spent much of Romanos' reign in retirement on his estates in Bithynia. It was here that he learned that his son Andronikos Doukas had joined and then deserted the emperor in the disastrous campaign ending with the Battle of Manzikert in 1071. Romanos' captivity gave John the opportunity to return to court at the request of Eudokia Makrembolitissa. Joining forces with Michael Psellos, the Caesar made the Empress share power with her son, and then forced her to become a nun and retire from court affairs in October 1071. He soon became the de facto head of the Government in the name of Michael VII, ordering the empire not to acknowledge Romanus as emperor, declaring that Romanos had been elevated to the throne to act for Michael, who was now able to administer the empire. The Caesar sent his sons Andronikos and Constantine to capture Romanos IV, who had been released from captivity and thus ensured the sole rule of his nephew Michael VII. John Doukas initially agreed to allow Romanos