John Komnenos Doukas was the son of Theodore Komnenos Doukas, first Emperor of Thessaloniki. When Theodore was captured and blinded by the Bulgarian tsar Ivan II Asen at the battle of Klokotnica in 1230, his brother Manuel seized the throne in Thessaloniki. In 1237 Ivan II Asen released Theodore, who chased his brother from Thessaloniki. He chose not to assume the imperial title himself, but crowned his son John as emperor. Theodore, however, controlled the state from his residence at Vodena in Macedonia. In 1242 John III Vatatzes of Nicaea advanced on Thessaloniki and captured Theodore. At this point John was persuaded to renounce his imperial title and accept the lesser dignity of despotes. John died two years later.
John's seal depicts the emperor in the company of St. Demetrios on the obverse and an inscription in eight lines on the reverse. John is shown wearing a crown and loros, and holding an akakia and a labarum. St. Demetrios is shown nimbate in military dress. The saint has his arm around the emperor and is gesturing towards the walled city of Thessaloniki, which has a building, perhaps the saint's shrine, in the center.