The Battle of Alesia or Siege of Alesia (September 52 BC) was a military engagement in the Gallic Wars around the Gallic oppidum (fortified settlement) of Alesia in modern France, a major centre of the Mandubii tribe. It was fought by the Roman army of Julius Caesar against a confederation of Gallic tribes united under the leadership of Vercingetorix of the Arverni. It was the last major engagement between Gauls and Romans, and is considered one of Caesar's greatest military achievements and a classic example of siege warfare and investment; the Roman army built dual lines of fortificationsan inner wall to keep the besieged Gauls in, and an outer wall to keep the Gallic relief force out. The Battle of Alesia marked the end of Gallic independence in the modern day territory of France and Belgium.

The battle site was probably atop Mont Auxois, above modern Alise-Sainte-Reine in France, but this location, some have argued, does not fit Caesar's description of the battle. A number of alternatives have been proposed over time, among which only Chaux-des-Crotenay (in Jura in modern France) remains a challenger today.The event is described by several contemporary authors, including Caesar himself in his Commentarii de Bello Gallico. After the Roman victory, Gaul (very roughly modern France) was subdued, although Gaul would not become a Roman province until 27 BC. The Roman Senate granted Caesar a thanksgiving of 20 days for his victory in the Gallic War.

Vercingetorix (Latin: [u̯ɛrkɪŋˈɡɛtɔriːks]; c. 82 – 46 BC) was a king and chieftain of the Arverni tribe who united the Gauls in a failed revolt against Roman forces during the last phase of Julius Caesar's Gallic Wars. Despite having willingly surrendered to Caesar, he was executed in Rome.

Vercingetorix was the son of Celtillus the Arvernian, leader of the Gallic tribes. Vercingetorix came to power after his formal designation as chieftain of the Arverni at the oppidum Gergovia in 52 BC. He immediately established an alliance with other Gallic tribes, took command, combined all forces and led them in the Celts' most significant revolt against Roman power. He won the Battle of Gergovia against Julius Caesar in which several thousand Romans and their allies were killed and the Roman legions withdrew.

Caesar had been able to exploit Gaulish internal divisions to easily subjugate the country, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. At the Battle of Alesia, also in 52 BC, the Romans besieged and defeated his forces; to save as many of his men as possible, he gave himself to the Romans. He was held prisoner for five years. In 46 BC, as part of Caesar's triumph, he was paraded through the streets of Rome and then executed by garroting. Vercingetorix is primarily known through Caesar's Commentarii de Bello Gallico (Commentaries on the Gallic War). To this day, he is considered a folk hero in Auvergne, his native region.