John I Doukas (), Latinized as Ducas, was an illegitimate son of Michael II Komnenos Doukas, Despot of Epirus in –1268. After his father's death, he became ruler of Thessaly from to his own death in 1289. From his father's family he is also inaccurately known as John Angelos. Married to a Thessalian Vlach woman, John first appears leading Vlach troops alongside his father in the lead-up to the Battle of Pelagonia in 1259. His defection to the camp of Emperor Michael VIII Palaiologos was crucial in the battle, which ended with the crushing defeat of the Epirotes' Latin allies and opened the way for the recovery of Constantinople and the re-establishment of the Byzantine Empire under Palaiologos in 1261. John quickly returned to the side of his father and brother, Nikephoros, and assisted them in recovering Epirus and Thessaly. After Michael II died, John Doukas became ruler of Thessaly with his seat at Neopatras, whence Western chroniclers often erroneously called him "Duke of Neopatras". Although Michael VIII Palaiologos engaged him in a marriage alliance and awarded him with the high title of sebastokratōr, John remained the foremost of Palaiologos' Greek opponents throughout the latter's reign. A staunch opponent of the Union of the Churches promoted by Palaiologos for political reasons, he provided refuge to several political opponents of the emperor, and even convoked synods that anathematized Palaiologos and the supporters of the Union. He resisted several attempts by Byzantine armies to conquer Thessaly, and allied himself with Palaiologos' Latin enemies, including the Duchy of Athens and Charles of Anjou. He died in 1289, leaving the rule of Thessaly to his sons, Constantine and Theodore. Origin and early life John Doukas was an illegitimate (and apparently the eldest) son of Michael II Komnenos Doukas, the Despot of Epirus, possibly by his one known mistress, an unnamed lady of the Gangrenos family. His full family name was Doukas Komnenos Angelos, but he is almost universally referred to in the sources simply by the first surname of "Doukas". This also led to a confusion among his Latin contemporaries, who mistook it for his title, and referred to him as the "Duke of Neopatras". His actual title, which he received from the Byzantine emperor Michael VIII Palaiologos in , was sebastokratōr. Nothing is known of his early life before 1259, when he is mentioned for his participation in the events that led up