Theoktistos or Theoctistus (; died November 20, 855) was a leading Byzantine official during the second quarter of the 9th century and the "de facto" head of the regency for the underage emperor Michael III from 842 until his dismissal and murder in 855. A eunuch courtier, he assisted in the ascent of Michael II to the throne in 822, and was rewarded with the titles of "patrikios" and later "magistros". He held the high posts of "chartoularios tou kanikleiou" and "logothetēs tou dromou" under Michael and his son Theophilos. After Theophilos' death in 842, Theoktistos became member of the regency council, but soon managed to sideline the other members and establish himself as the virtual ruler of the Empire. Noted for his administrative and political competence, Theoktistos played a major role in ending the Byzantine Iconoclasm, and fostered the ongoing renaissance in education within the Empire. He also continued the persecution of the Paulician sect, but had mixed success in the wars against the Arabs. When Michael III came of age in 855, his uncle Bardas persuaded him to throw off the tutelage of Theoktistos and his mother, the Empress-dowager Theodora, and on 20 November 855, Theoktistos was assassinated by Bardas and his followers.