Environmental niche models predict the presence of the invasive Argentine ant in north-western Europe, especially along all the French Atlantic coast. Yet, the species has never been observed North from the 45th parallel in Europe, suggesting either that current models are wrong or that Argentine ants are already spreading north inconspicuously. Here, we report a 3-hectare wide colony of Argentine ants, detected in 2016 in Nantes, France, which is 300 km north of the former northernmost outdoor population of this species in Europe. COI sequencing revealed that the haplotype of this new colony is the same as the one found in the so-called Catalonian supercolony, which is distinct from the haplotype found over most of the species range in Europe. Our discovery confirms models’ predictions that Argentine ants can colonize north-western Europe and suggests that they might have already reached several other locations along the French Atlantic coast. Detection surveys should be conducted to assess Argentine ants’ invasion patterns in Western France, particularly in high introduction risk areas such as major cities and maritime ports.