Crassostrea gigas originated from the Pacific coast of Asia but was introduced into several European countries in the early 1970s. Natural populations have now spread the length of the western seaboard of Europe. To elucidate the demographic and selective processes at play during this rapid expansion, genome-scan analysis was performed on different populations. High diversities and low differentiation were observed overall, but significant genetic differentiation was found among newly-established populations and between the newly-established northern group and a nearly panmictic group composed of southern European populations and a population from Japan. Loss of genetic diversity was also seen in the north, likely caused by founder events during colonisation. The few strongly supported outlier loci revealed a genetic structure uncorrelated with the north/south differentiation, but grouping two samples from the Danish Fjords (northern group) and one from the Dutch Scheldt estuary (southern group) with the one from Japan. These findings might reflect: (i) parallel adaptation to similar environmental pressures (Fjord-like environment) within each of the two groups or (ii) a footprint of a secondary introduction of an alternative genomic background maintained by multifarious isolation factors. Our results call for a closer examination of adaptive genetic structure in the area of origin.