The distribution range of the American comb jelly Mnemiopsis leidyi has expanded across Europe for several decades, particularly in the eastern Atlantic and Mediterranean Sea. This study aims to assess its expansion along the French Atlantic coast, mainly in the Bay of Biscay and Iroise Sea, since the first record in 2014. Mnemiopsis leidyi is now clearly established along 500 km of coastline, from the coast of Lorient to Arcachon Bay, which is the southernmost colonised area in the eastern Atlantic to date. It will likely colonise the Spanish Gulf coast in the near future through natural dispersal via currents, as has occurred between the Gironde estuary and Arcachon Bay. We quantify that this species now colonises nearly 45% of the French coastline. The invaded area includes the main estuaries of the Seine, Loire, Gironde and Rhône rivers, where M. leidyi populations may constitute reservoirs for colonising other harbours through merchant vessel traffic via ballast water. Finally, the Marennes- Oléron Bay and Arcachon Bay are the two main spat-producing regions for the Pacific oyster Magallana gigas in France. As M. leidyi consumes bivalve larvae, the potential economic and ecological impacts on this shellfish industry are discussed.