Dumnorix (d.54 B.C.) was a leader of the anti-Roman faction in the Aedui tribe, and the brother of the pro-Roman leader Divitiacus. In or around 61 B.C. Dumnorix began to plot with the Helvetian nobleman Orgetorix, who was planning to lead his tribe in a migration from modern Switzerland to the west coast of France. Dumnorix married Orgetorix's daughter, and remained a supporter of the migration even after Orgetorix died soon after his plot to seize power from the Helvetian magistrates was discovered.
When the migration began Caesar refused to give the Helvetii permission to cross the Roman province of Transalpine Gaul. Dumnorix convinced the Sequani to let the Helvetii cross their lands instead, but by the time the migration reached the Saone the Aedui were serving as allies of Caesar, and Dumnorix found himself commanding the Aedui cavalry in the campaign against his own personal allies.
Caesar defeated one part of the Helvetii horde as they were crossing the Saone (battle of the Arar). After an unsuccessful meeting between Caesar and the Helvetii leaders the migration continued. Dumnorix and the Aedui cavalry were in the Roman vanguard. On the day after the failed meeting the Roman and Aedui cavalry were driven off by the Helvetii rearguard, probably because Dumnorix led his men away from the battle as quickly as was credibly possible.
Over the next fifteen days the Romans followed the Helvetii. They were soon forced to abandon their own source of supplies, and had to rely on the Aedui to supply them with corn. Dumnorix made sure that no supplies reached the Romans, but eventually his plotting was discovered. Caesar wanted him punished, but Divitiacus managed to convince Caesar to forgive his brother. Dumnorix retained his position, but for the rest of the campaign against the Helvetii was carefully watched.
Despite his hostility to the Romans Dumnorix remained a powerful figure. He was popular, wealth and commanded a large force of his own cavalry. His mother was married to an important man in the Biruriges and his other female relatives were married into other tribes, giving him influse across Gaul. He apparently hated the Romans because Caesar's arrival in Gaul had reduced his power and increased that of his brother Divitiacus. Caesar also stated that Dumnorix believed that if the Aedui could not retain independence then they would have been better off under the Helvetii the Romans.
In 54 BC, as Caesar prepared to cross to England, he decided to take most of the Gallic aristocracy with him to make sure that Gaul remained peaceful in his rear. Dumnorix had managed to regain some power amongst the Aeduans, and was unwilling to accompany the army. He was forced to join Caesar on the north coast, where poor weather delayed the sailing for twenty-five days. During this period Dumnorix attempted to subvert his fellow Gallic leaders, claiming that Caesar was planning to take them to Britain to kill them. These clearly failed, for on the day that the weather improved and the fleet prepared to sail Dumnorix fled from the camp at the head of the Aeduan cavalry. Caesar sent the rest of his cavalry in pursuit. Dumnorix was caught and killed while attempting to resist capture.