Starting from the second century BC, with the fast expansion of the Roman Empire, iron production and consumption developed exponentially in north-western Europe. This rapid growth naturally led to an increase in trade, that still remains to be studied encompassing a broad scope, so as to not neglect long-distance exchanges. This is today possible by taking advantage of the progress made in the past 40 years in archaeology and archaeometallurgy. Cargoes of iron bars recovered from a group of 23 wrecks located off the coast of Saintes-Maries-de-la-Mer (Bouches-du-Rhône, France), opposite an old branch of the Rhône River, constitute a rich opportunity to examine this trade, by comparing the slag inclusions trapped in iron bars to primary slag from the six main ironmaking areas in Gaul. Based on a trace element analysis of these inclusions and this slag, we suggest that ships travelled down the Rhône carrying iron produced in Wallonia (Belgium), while others sailed up the Rhône transporting iron produced in Montagne Noire (Aude, France)., Competing Interests: The authors have declared that no competing interests exist.