Gaul was divided into several different provinces. The Romans displaced populations to prevent local identities from becoming a threat to Roman control. Thus, many Celts were displaced in Aquitania or were enslaved and moved out of Gaul. There was a strong cultural evolution in Gaul under the Roman Empire, the most obvious one being the replacement of the Gaulish language by Vulgar Latin. It has been argued the similarities between the Gaulish and Latin languages favoured the transition. Gaul remained under Roman control for centuries and Celtic culture was then gradually replaced by Gallo-Roman culture.

The Gauls became better integrated with the Empire with the passage of time. For instance, generals Marcus Antonius Primus and Gnaeus Julius Agricola were both born in Gaul, as were emperors Claudius and Caracalla. Emperor Antoninus Pius also came from a Gaulish family. In the decade following Valerian's capture by the Persians in 260, Postumus established a short-lived Gallic Empire, which included the Iberian Peninsula and Britannia, in addition to Gaul itself. Germanic tribes, the Franks and the Alamanni, entered Gaul at this time. The Gallic Empire ended with Emperor Aurelian's victory at Châlons in 274.

A migration of Celts appeared in the 4th century in Armorica. They were led by the legendary king Conan Meriadoc and came from Britain. They spoke the now extinct British language, which evolved into the Breton, Cornish, and Welsh languages.

In 418 the Aquitanian province was given to the Goths in exchange for their support against the Vandals. Those same Goths had sacked Rome in 410 and established a capital in Toulouse.

The Roman Empire had difficulty responding to all the barbarian raids, and Flavius Aëtius had to use these tribes against each other in order to maintain some Roman control. He first used the Huns against the Burgundians, and these mercenaries destroyed Worms, killed king Gunther, and pushed the Burgundians westward. The Burgundians were resettled by Aëtius near Lugdunum in 443. The Huns, united by Attila, became a greater threat, and Aëtius used the Visigoths against the Huns. The conflict climaxed in 451 at the Battle of Châlons, in which the Romans and Goths defeated Attila.

The Roman Empire was on the verge of collapsing. Aquitania was definitely abandoned to the Visigoths, who would soon conquer a significant part of southern Gaul as well as most of the Iberian Peninsula. The Burgundians claimed their own kingdom, and northern Gaul was practically abandoned to the Franks. Aside from the Germanic peoples, the Vascones entered Wasconia from the Pyrenees and the Bretons formed three kingdoms in Armorica: Domnonia, Cornouaille and Broërec.