Constantine X Doukas or Dukas, Latinized as Ducas (Greek: Κωνσταντῖνος Ι΄ Δούκας) was emperor of the Byzantine Empire from 24 November 1059 to 22 May 1067.
Constantine Doukas was the son of Andronikos Doukas, a Paphlagonian nobleman who may have served as governor of the theme of Moesia. In 1057, Constantine supported the usurpation of Isaac I Komnenos, gradually siding with the court bureaucracy against the new emperor's reforms. In spite of this tacit opposition, Constantine was chosen as successor by the ailing Isaac in November 1059, under the influence of Michael Psellos. Isaac abdicated, and on November 24, 1059, Constantine X Doukas was crowned emperor.
Already old and unhealthy when he came to power, Constantine died on May 22, 1067. His final act was to demand that only his sons succeed him, forcing his wife Eudokia Makrembolitissa to take a vow not to remarry.