Aquitaine (UK: /ˌækwɪˈteɪn/, US: /ˈækwɪteɪn/; French: [akitɛn] ^ⓘ; Occitan: Aquitània [akiˈtanjɔ]; Basque: Akitania; Poitevin-Saintongeais: Aguiéne), archaic Guyenne or Guienne (Occitan: Guiana), is a historical region of southwestern France and a former administrative region. Since 1 January 2016 it has been part of the administrative region of Nouvelle-Aquitaine. It is situated in the southwest corner of metropolitan France, along the Atlantic Ocean and the Pyrenees mountain range on the border with Spain; for most of its written history Bordeaux has been a vital port and administrative centre. It is composed of the five departments of Dordogne, Lot-et-Garonne, Pyrénées-Atlantiques, Landes and Gironde. Gallia Aquitania was established by the Romans in ancient times and in the Middle Ages, Aquitaine was a kingdom and a duchy, whose boundaries fluctuated considerably.

There are traces of human settlement by prehistoric peoples, especially in the Périgord, but the earliest attested inhabitants in the south-west were the Aquitani, who were not considered Celtic people, but more akin to the Iberians (see Gallia Aquitania). Although a number of different languages and dialects were in use in the area during ancient times, it is most likely that the prevailing language of Aquitaine during the late pre-historic to Roman period was an early form of the Basque language. This has been demonstrated by various Aquitanian names and words that were recorded by the Romans, and which are currently easily readable as Basque. Whether this Aquitanian language (Proto-Basque) was a remnant of a Vasconic language group that once extended much farther, or it was generally limited to the Aquitaine/Basque region, is not known. One reason the language of Aquitaine is important is because Basque is the last surviving non-Indo-European language in western Europe and it has had some effect on the languages around it, including Spanish and, to a lesser extent, French.

The original Aquitania (named after the inhabitants) at the time of Caesar's conquest of Gaul included the area bounded by the river Garonne, the Pyrenees and the Atlantic Ocean. The name may stem from Latin 'aqua', maybe derived from the town "Aquae Augustae", "Aquae Tarbellicae" or just "Aquis" (Dax, Akize in modern Basque) or as a more general geographical feature.

Under Augustus' Roman rule, from 27 BCE the province of Aquitania was further stretched to the north to the river Loire, thus including proper Gaul tribes along with old Aquitani south of the Garonne (cf. Novempopulania and Gascony) within the same region.

In 392, the Roman imperial provinces were restructured as Aquitania Prima (north-east), Aquitania Secunda (centre) and Aquitania Tertia, better known as Novempopulania in the south-west.

Accounts of Aquitania during the Early Middle Ages are imprecise, but there was much unrest. The Visigoths were called into Gaul as foederati, legalizing their status within the Empire. Eventually they established themselves as the de facto rulers in south-west Gaul as central Roman rule collapsed. Visigoths established their capital in Toulouse, but their tenure on Aquitaine was feeble. In 507, they were expelled south to Hispania after their defeat in the Battle of Vouillé by the Franks, who became the new rulers in the area to the south of the Loire.

The Roman Aquitania Tertia remained in place as Novempopulania, where a duke was appointed to hold a grip over the Basques (Vascones/Wascones, rendered Gascons in English). These dukes were quite detached from central Frankish overlordship, sometimes governing as independent rulers with strong ties to their kinsmen south of the Pyrenees. As of 660, the foundations for an independent Aquitaine/Vasconia polity were established by the duke Felix of Aquitaine, a magnate (potente(m)) from Toulouse, probably of Gallo-Roman stock. Despite its nominal submission to the Merovingians, the ethnic make-up of the new Aquitanian realm was not Frankish, but Gallo-Roman north of the Garonne and in main towns and Basque, especially south of the Garonne.

A united Basque-Aquitanian realm reached its heyday under Odo the Great's rule. In 721, the Aquitanian duke fended Umayyad troops (Sarracens) off at Toulouse, but in 732 (or 733, according to Roger Collins), an Umayyad expedition commanded by Abdul Rahman Al Ghafiqi defeated Odo next to Bordeaux, and went on to loot its way up to Poitiers. Odo was required to pledge allegiance to the Frankish Charles Martel in exchange for help against the advancing Arab forces. Basque-Aquitanian self-rule temporarily came to a halt, definitely in 768 after the assassination of Waifer.