The Duchy of Aquitaine is a historical fiefdom in what is now southwestern France, corresponding roughly to the French region of Aquitaine, although its extent fluctuates greatly over the centuries, at times comprising much of what is now southwestern and central France, at others reduced to the area of Gascony on the southwestern fringes of France.It originates in the 7th century as a duchy under Frankish suzerainty, ultimately a continuation of the Roman province of Gallia Aquitania.

It becomes a "subkingdom" within the Carolingian Empire and in the High Middle Ages fell to the kings of England, whose claims in France trigger the Hundred Years' War.

The kingdom of France emerges victorious from this conflict in the 1450s, and the duchy of Aquitaine is incorporated under the direct rule of the French kings.