Armorica or Aremorica is the name given in ancient times to the part of Gaul that includes the Brittany peninsula and the territory between the Seine and Loire rivers inland to an indeterminate point.

Pliny the Elder, in his Natural History (2.17.105), claims that Armorica was the older name for Aquitaine, stating Armorica's southern boundary extended to the Pyrenees. Pliny lists the following Celtic tribes as living in the Brittany peninsula: the Aedui and Carnuteni as having treaties with Rome; the Neldi and Secusiani as having some measure of independence; and the Boii, Senones, Aulerci (both the Eburovices and Cenomani ), the Parisii, Tricases , Andicavi , Viducasses, Bodiocasses , Venelli , Coriosvelites , Diablinti , Rhedones , Turones , and the Atseui .

Because continued resistance to Roman rule in Armorica was supported by Celtic aristocrats in Britain, Julius Caesar led two invasions of Britain in 55 and 54 BC in response.

Under the Roman Empire, Armorica was administered as part of the province of Lugdunensis, which had its capital in Lyons. When the Roman provinces were reorganized in the 4th century, Armorica was placed under the second and third Lugdunensium provinces. Armorica rebelled from the Roman Empire in the 430s and again in the 440s, throwing out the ruling officials.

As the Brittany peninsula came to be settled with Romano-British from Britain in the 5th through 7th centuries, and Vikings or Northmen settled in the Cotentin peninsula and the lower Seine around Rouen in the 9th and 10th centuries, these regions came to be known as Brittany and Normandy respectively, and the name Armorica fell out of use.