Demetrius or Demetrios of Montferrat (; , Dēmētrios Momferratikos; 1205–1230) was King of Thessalonica from 1207 to 1224. Life Demetrius was the son of Marquis Boniface of Montferrat by Margaret of Hungary, the widow of Byzantine Emperor Isaac II Angelos. In the aftermath of the Fourth Crusade, Boniface had secured for himself the kingdom of Thessalonica in the division of the spoils. Marrying the former Byzantine empress, he found it easier to attract support from the members of the Byzantine aristocracy. When Boniface's son was born in Thessalonica in 1205, he was named after Saint Demetrius, the patron saint of Thessalonica. Boniface was killed in a battle against the Bulgarians in 1207, and Kaloyan of Bulgaria promptly besieged Thessalonica. The siege ended with Kaloyan's murder, but Demetrius' rule was not secure. The most influential magnates of the kingdom conspired against him, plotting to replace the infant ruler with his half-brother, Marquis William VI of Montferrat. The Latin Emperor Henry of Flanders determined to stem this movement and advanced on Thessalonica in December 1208. The barons, led by Count Umberto II of Biandrate, shut the gates of the city before the emperor and issued several unreasonable demands. The emperor pretended to accept Biandrate's terms (provided that Margaret accepted them) long enough to gain entrance into the city. Margaret was then instructed to overrule the terms and Henry crowned young Demetrius king on January 6, 1209, extracting oaths of homage from the Lombard barons. The emperor's actions were confirmed by Pope Innocent III. The rights of the young king were still threatened by Biandrate's Lombard garrisons of important fortresses like Serres and Kavala. Henry took Serres but Kavala held out. Hoping to reach a reconciliation, the emperor summoned the nobility of the Kingdom of Thessalonica to a parliament at Ravennika, but many lords did not submit. After further military action, Henry obtained the submission of Biandrate and the remaining Lombard lords. He defeated the Epirote attack on Thessalonica in 1210 and left his younger brother Eustace as regent for King Demetrius. The situation changed again on Henry's death in 1216. The new Latin Emperor, Peter of Courtenay, was won over by the Lombard lords and invested William of Montferrat with the kingdom before leaving Italy. The Queen Mother Margaret fled to Hungary, but Demetrius apparently remained in place, while the Emperor Peter was captured and eventually executed by Theodore Komnenos Doukas in