The Duchy of Gascony or Duchy of Vasconia (Basque: Baskoniako dukerria; Occitan: ducat de Gasconha; French: duché de Gascogne, duché de Vasconie) was a duchy in present southwestern France and northeastern Spain, part corresponding to the modern region of Gascony after 824. The Duchy of Gascony, then known as Wasconia, was originally a Frankish march formed to hold sway over the Basques (Vascones). 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.

In the Hundred Years' War, Charles V of France conquered most of Gascony by 1380, and under Charles VII of France it was incorporated into the kingdom of France in its entirety in 1453. The corresponding portion within Spain became part of the Basque Kingdom of Navarre.

Gascony was the core territory of Roman Gallia Aquitania. This province, by the 2nd century, was extended to include much of western Roman Gaul, as far north as the Loire. Thus, the name of the Aquitani came to be transferred to the territory of central-western France later known as the Duchy of Aquitaine. In 293, Diocletian re-created the original province of Caesar's Aquitania under the name of Novempopulania or Aquitania Tertia.

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