William VI of Saint Omer was the castellan of Saint-Omer and lord of Fauquembergues briefly in ca. 1246/7.

The genealogist Baldwin of Avesnes records him as a son of William V of Saint Omer, but Arthur Giry has him rather a younger brother of the latter, i.e. he was one of the eleven children of William IV of Saint Omer and Ida of Avesnes.^[1] He is first attested in April 1207, and appears thereafter in various charters and documents, in which from 1218 on he appears as lord of Pitgam and later of Berkin.^[2] He succeeded his brother sometime between March 1246 and his own only attestation as castellan, in August 1247. Very little is known of his tenure, which apparently was very brief; Baldwin of Avesnes merely records that he went on Crusade, where he died. He was succeeded by his eldest sister Beatrice and her son William of Renenghes by 1251.^[3]