At last, the Roman fleet sailed, and encountered the Venetic fleet off the coast of Brittany in the Gulf of Morbihan. They engaged in a battle that lasted from late in the morning until sundown. On paper, the Veneti appeared to have the superior fleet. Their ships' sturdy oak beam construction meant they were effectively immune to ramming, and their high-profile protected their occupants from projectiles. The Veneti had some 220 ships, although Gilliver notes many were likely not much more than fishing boats. Caesar did not report the number of Roman ships. The Romans had one advantage—grappling hooks. These allowed them to shred the rigging and sails of the Venetic ships that got close enough rendering them inoperable. The hooks also allowed them to pull ships close enough to board. The Veneti realized the grappling hooks were an existential threat and retreated. However, the wind dropped, and the Roman fleet (which did not rely on sails) was able to catch up. The Romans could now use their superior soldiers to board ships en masse and overwhelm the Gauls at their leisure. Just as the Romans had beaten the superior forces of Carthage in the First Punic War by using the corvus boarding device, a simple technological advantage—the grappling hook—allowed them to defeat the superior Venetic fleet.

The Veneti, now without a navy, had been bested. They surrendered, and Caesar made an example of the tribal elders by executing them. He sold the rest of the Veneti into slavery. Caesar now turned his attention to the Morini and Menapii along the coast.