1246) was the castellan of Saint-Omer from 1192 until his death, as well as Lord of Beaurain and Fauquembergues.
1171, he was the eldest son of William IV of Saint Omer and Ida of Avesnes.
[1] He is attested in numerous charters and deeds of property as well as donation to the Church over the next decades.
He repeatedly pledged his loyalty to King Philip Augustus during the latter's conflicts with his neighbours, and is occasionally mentioned among the Flemish feudatories of the French court.
[3] The marriage was childless, and after his death (sometime between March 1246 and August 1247) he was succeeded by his sole surviving brother, William VI of Saint Omer.