The Duchy of Gascony or Duchy of Vasconia was a duchy located in present-day southwestern France and northeastern Spain, an area encompassing the modern region of Gascony. The Duchy of Gascony, then known as Wasconia, was originally a Frankish march formed to hold sway over the Basques. However, the duchy went through different periods, from its early years with its distinctively Basque element to the merger in personal union with the Duchy of Aquitaine to the later period as a dependency of the Plantagenet kings of England.

A duchy, also called a dukedom, is a medieval country, territory, fief, or domain ruled by a duke or duchess, a ruler hierarchically second to the king or queen in Western European tradition.