Sancho VI William (Basque: Antso Gilen, French: Sanche Guillaume, Gascon: Sans Guilhem, Spanish: Sancho Guillén) (died 4 October 1032) was the Duke of Gascony from 1009 to his death.
His reign is most notable for the renewal of Gascons ties with Spain.
Sancho was a son of William II Sánchez and Urraca of Navarre and relative of Sancho III of Navarre and he spent a portion of his life at the court of that king in Pamplona.
In 1010, he appeared together with Sancho, Robert II of France, and William V of Aquitaine at Saint-Jean d'Angély.
In 1027, he met William V at Blaye and they jointly selected Geoffrey, a Frank, as Archbishop of Bordeaux, which had become the Gascon capital during Sancho's reign.