The Duke of Aquitaine was the ruler of the medieval region of Aquitaine under the supremacy of Frankish, English, and later French kings.

The Duchy of Aquitaine was a historical fiefdom in western, central, and southern areas of present-day France to the south of the river Loire, although its extent, as well as its name, fluctuated greatly over the centuries, at times comprising much of what is now southwestern France (Gascony) and central France.