Ruler of Thessalonica (1207-1224), the only member of a Frankish ruling dynasty in Greece to be named after a Greek saint. Demetrius was the son of Boniface of Montferrat and Maria (Margaret) of Hungary. He succeeded his father as lord of Thessalonica (mod. Thessaloniki, Greece) in September 1207; his mother acted as regent together with Oberto of Biandrate (1207-1211) and Berthold of Katzenelnbogen (after 1211).
Oberto, along with many important Lombard lords, the Templars, and the Venetian interests in the area, sought to replace Demetrius with his half-brother William VI of Mont- ferrat, who, as an adult and a warrior, had the qualities necessary to defend the area from Greek attacks from Epiros. Demetrius and his mother were backed by the Latin emperor, Henry, who marched to Thessalonica in December 1208. On 6 January 1209, Henry crowned Demetrius the first king of Thessalonica, an action that was endorsed by Pope Innocent III in March 1209 when he took the infant under papal protection.
Whatever this action did to remove any threat to the Latin Empire from the house of Montferrat, it did little to save the kingdom of Thessalonica from Greek attack. By 1222 the city was under threat from Theodore Doukas of Epiros. Demetrius went to Italy to seek military support from Pope Honorius II and William VI of Montferrat, leaving Guy Pallavicini of Boudonitza in charge of the defense of the city. During Demetrius’s absence, the situation in Thessalonica became hopeless; his mother returned to Hungary in 1223, and in December 1224 the city surrendered to the Epirotes. The following year, Demetrius and his half-brother mounted a campaign through Thessaly to recapture the city, but the army broke up following the death of William VI, and Demetrius retired to Pavia.
In 1228 Demetrius is recorded in the entourage of Emperor Frederick II at the “Feast of Kings” at Nicosia in Cyprus. He died in 1230, having bequeathed his titular kingship to Frederick II.