John I Doukas of Thessaly

John I Doukas (c. 1231–1289) was an illegitimate son of Michael II Komnenos Doukas, Despot of Epirus, who ruled Thessaly from 1268 until his death. In 1259 he led Vlach troops before defecting to Michael VIII Palaiologos at Pelagonia, helping the Nicaeans crush the allies; he then returned to help his family recover Epirus and Thessaly.

He became the emperor's foremost Greek opponent, rejecting the Union of the Churches, sheltering its enemies and allying with Athens and Charles of Anjou; imperial armies never subdued him. Made sebastokrator around 1272, he died in 1289, leaving Thessaly to his sons Constantine and Theodore.