William V of Saint Omer

William of Saint Omer was a French knight, descended from a Fauquembergues family who were castellans of the eponymous castle of Saint-Omer.

His father, Nicholas I of Saint Omer, received lands in Boeotia in the aftermath of the Fourth Crusade. He later married Margaret of Hungary, the widow of Boniface of Montferrat, Lord of Thessalonica (died 1207). It is unclear when the marriage took place: some accounts mention that Nicholas died already in 1212 and J. Longnon "not before 1223".

William was the younger son of the couple, he had an elder brother Bela of Saint Omer.

Béla IV betrothed William to his second eldest daughter Margaret (not to be confused with Saint Margaret of Hungary), however both persons died by 1242. According to his epitaph at the Cathedral of Saint Domnius, William died in April 1242 in Trogir, where the royal family resided until the Mongol occupation.