The green seaweeds Ulva australis and U. pertusa were described from southern Australia and Japan, respectively. They are conspecific and U. australis, the currently accepted taxon, is native to temperate marine waters in north-eastern Asia, and known to be introduced overseas into Australasia, the Americas and Europe. Although the genetics of U. australis have been investigated elsewhere, along French coasts the origins and history of the introduction of this species need to be clarified. We used mitochondrial, plastid and nuclear markers to differentiate introduced populations of U. australis along the French Atlantic coasts. The plastid tufA gene used as a barcoding marker revealed a well-defined species with a higher haplotype diversity in native vs. introduced areas. The ITS2 region (nuclear) and rbcL (plastid) were used to compare French specimens with the lectotype of U. australis. Putative geographic origins of the genetically determined U. australis were examined using genetic markers with better resolution, the plastid atpI-H combined with the mitochondrial trnA-N. Origin (s) and introduction history of French specimens were inferred from the comparison between their haplotypes and those previously described in native and non-native temperate areas worldwide. Our results indicate that the presence of U.australis along the French Atlantic and Mediterranean coasts is the result of multiple introductions and independent pathways, and suggest that historical oyster transfers from Japan and British Columbia can only partially explain the observed patterns in genetic markers. Alternative hypotheses for the timing and pathways of introductions are proposed in the light of the historic background of maritime transport networks and trade between north-eastern Asia and Europe.