Informed by deserters from the allied auxiliary cavalry of Lucius Aemilius (the commander of the cavalry), the Helvetii decided to harass Caesar's rear guard. When Caesar observed this, he sent his cavalry to delay the attack. He then placed the Seventh (Legio VII Claudia), Eighth (Legio VIII Augusta), Ninth (Legio IX Hispana), and Tenth legions (Legio X Equestris), organized in Roman fashion (triplex acies, or "triple battle order"), at the foot of a nearby hill, the top of which he occupied himself, along with the Eleventh (Legio XI Claudia) and Twelfth (Legio XII Fulminata) Legions and all his auxiliaries. His baggage train was assembled near the summit, where it could be guarded by the forces there.

Having driven off Caesar's cavalry and with their own baggage train secured, the Helvetii engaged "In the seventh hour", approximately noon or one o'clock. According to Caesar, his hilltop battle line easily threw back the onslaught by using pila (javelins/throwing spears). The Roman legionaries then drew swords and advanced downhill wading into their opponents. Many Helvetii warriors had pila sticking out of their shields and threw them aside to fight unencumbered, but this also made them more vulnerable. The legions drove the Helvetii back toward the hill where their baggage train sat.

While the legions pursued the Helvetii across the plain between the hills, the Boii and the Tulingi arrived with fifteen thousand men to assist the Helvetii, flanking the Romans on one side. At that point, the Helvetii returned to the battle in earnest. When the Tulingi and the Boii started circumventing the Romans, Caesar regrouped his third line to resist the assault of the Boii and Tuligni, keeping his primary and secondary committed to chasing the Helvetii.

The battle lasted many hours into the night, until the Romans finally took the Helvetic baggage train, capturing both a daughter and a son of Orgetorix. According to Caesar, 130,000 enemies escaped, of which 110,000 survived the retreat. Unable to pursue on account of battle wounds and the time it took to bury the dead, Caesar rested three days before he followed the fleeing Helvetii. These, in turn, had managed to reach the territory of the Lingones within four days of the battle. Caesar warned the Lingones not to assist them, prompting the Helvetii and their allies to surrender.