The Duchy of Aquitaine (Occitan: Ducat d'Aquitània, IPA: [dyˈkad dakiˈtaɲɔ]; French: Duché d'Aquitaine, IPA: [dyʃe dakitɛn]) was a historical fiefdom located in the western, central and southern areas of present-day France, south of the river Loire.

Although the full extent of the duchy, as well as its name, fluctuated greatly over the centuries and at times comprised much of what is now southwestern (Gascony) and central France.

The Hundred Years' War finally saw the kingdom of France gain full control over Aquitaine in the 1450s, with much of its territory directly incorporated into the French royal domain itself.

During the 6th and early 7th century, it was under direct rule of Frankish kings, divided between the realms of Childebert II and Guntram in the Treaty of Andelot of 587.

Charibert campaigned successfully against the Basques, but after his death in 632, they revolted again, in 635 subdued by an army sent by Dagobert (who was at the same time forced to deal with a rebellion in Brittany).

The duchy of Aquitaine established itself as a quasi-independent realm within the Frankish empire during the second half of the 7th century, certainly by 700 under Odo the Great.

Odo was succeeded by his son Hunald, who reverted to former independence, so defying the Frankish Mayor of the Palace Charles Martel's authority.

Waifer strenuously carried on an unequal struggle with the Carolingian Franks, but his assassination in 768 marked the demise of Aquitaine's relative independence.

Particularly with the Liber Judiciorum, which was codified in 642 and 643 and expanded in the Code of Recceswinth in 653, women could inherit land and title and manage it independently from their husbands or male relations, dispose of their property in legal wills if they had no heirs, and women could represent themselves and bear witness in court by age 14 and arrange for their own marriages by age 20.

By a treaty made in 845 between Charles the Bald and Pepin II, the kingdom had been diminished by the loss of Poitou, Saintonge and Angoumois in the northwest of the region, which had been given to Rainulf I, count of Poitiers.

William's duchy almost reached the limits of the old Roman Gallia Aquitania but did not stretch south of the Garonne, a district which was in the possession of the Gascons.

His granddaughter, Eleanor of Aquitaine, succeeded to the duchy at the age of 15 as the eldest daughter and heir of William X (d. 1137), as his son did not live past childhood.

The marriage was later annulled on the grounds of consanguinity by a bishop on 21 March 1152, and she kept her lands and title as Duchess of Aquitaine in her own right.

When Richard died in 1199, it reverted to Eleanor, and on her death in 1204, it was inherited by her son John and absorbed into the English crown permanently.

Aquitaine as it came to the English kings stretched from the Loire to the Pyrenees, but its range was limited to the southeast by the extensive lands of the counts of Toulouse.

In 1337, King Philip VI of France reclaimed the fief of Aquitaine (essentially corresponding to Gascony) from Eleanor's descendant, Edward III of England.

In 1360, both sides signed the Treaty of Brétigny, in which Edward renounced his claim to the French crown but remained sovereign lord of Aquitaine (rather than merely duke).

In 1390, King Richard II, son of Edward the Black Prince, appointed his uncle John of Gaunt as Duke of Aquitaine.

That title passed on to John's descendants although they belonged to the crown because John of Gaunt's son, Henry IV, managed to successfully usurp the crown from Richard II, therefore 'inheriting' the title Lord of Aquitaine from his father, which was passed down to his descendants as they became Kings.

The Valois kings of France, claiming supremacy over Aquitaine, granted the title of duke to their heirs, the Dauphins, during 1345 and 1415: John II (1345–50), Charles VII (1392?–1401), and Louis (1401–1415).