Boniface I, usually known as Boniface of Montferrat (Italian: Bonifacio del Monferrato; Greek: Βονιφάτιος Μομφερρατικός, Vonifatios Momferratikos), was Marquess of Montferrat (from 1192), the leader of the Fourth Crusade (1201-04) and the King of Thessalonica (from 1204).
Boniface was the third son of William V of Montferrat and Judith of Babenberg, born after his father's return from the Second Crusade. He was a younger brother of William "Longsword", Count of Jaffa and Ascalon, and of Conrad I of Jerusalem.
When the original leader of the Fourth Crusade, Count Theobald III of Champagne, died in 1201, Boniface was chosen as its new leader. He was an experienced soldier, and it was an opportunity to reassert his dynasty's reputation after defeat at home. Boniface's family was well known in the east: his nephew Baldwin and brother Conrad had been Kings of Jerusalem, and his niece Maria was heiress of the kingdom.
After the conquest of Constantinople in 1204, Boniface was assumed to be the new emperor, both by the western knights and the conquered Byzantine citizens. However, the Venetians vetoed him, believing that he already had too many connections in the Empire (and, likely, felt that they would not have as much influence in the new Empire if Boniface was in control). Instead, they chose Baldwin of Flanders. Boniface founded the Kingdom of Thessalonica and also held all the territories lied east of Bosphorus and territories in Crete, though he later conceded Crete to Baldwin. Late 13th and 14th century sources suggest that Boniface based his claim to Thessalonica on the statement that his younger brother Renier had been granted Thessalonica on his marriage to Maria Komnene in 1180.
Boniface was killed in an ambush by the Bulgarians on 4 September 1207, and his head was sent to Bulgarian Tsar Kaloyan.