William V of Aquitaine (969 – January 30, 1030), nicknamed le Grand (the Great), was Duke of Aquitaine and Count of Poitiers as William II of Poitiers.

William was the son of William IV of Aquitaine by his wife Emma of Blois. He seems to have taken after his formidable mother, who ruled Aquitaine until 1004. He was a friend to Fulbert of Chartres and founded a cathedral school at Poitiers. He himself was very well educated, a collector of books, and turned the prosperous court of Aquitaine into the learning center of Southern France.

He had three marriages:
• Agnes of Gévaudan, widow of Aldebert I of La Marche
1. William VI of Aquitaine

• Prisca (a.k.a. Brisque) of Gascony, daughter of Duke Sans VI Guilhem of Gascony. She was dead by 1018.
1. Eudes of Aquitaine
2. Adalais of Aquitaine, married Count Guiraut I Trancaleon of Armagnac
3. Thibault, died young.

• Agnes of Burgundy, daughter of Otto-William, Duke of Burgundy. Her second husband was Geoffrey II of Anjou.
1. Pierre-Guillaume (William VII)
2. Guy-Geoffroy (William VIII)
3. Agnes de Poitou, married Henry III, Holy Roman Emperor