The Atlantic rock crab (Cancer irroratus) was first recorded in Iceland in 2006 and has since rapidly spread throughout the country´s southwestern and western coastal waters. The transport of larvae in ballast water is regarded as the most probable means by which introduction into Iceland occurred. As this species is commercially valuable, it may be possible to establish a viable industry harvesting rock crabs in Iceland, however to do this more information on species wide genetic diversity and demography is required. In this study genetic variation at seven microsatellite markers was analysed in samples from Iceland and five sites in North America, capturing most of the known range of this species. Our results divided samples from the native range into two groups, divided by a previously proposed barrier to gene flow, compatible with local hydrographic factors restricting larval-mediated gene flow. The Icelandic population was markedly differentiated from all other samples, but exhibited comparable levels of genetic diversity with no evidence of small population effects or genetic bottlenecks. No single population could be identified as a source for the Icelandic population. Genetic data indicate that the number of founders of the Icelandic population was sufficient to retain genetic variation. As the Icelandic population shows evidence of self-recruitment and population expansion it may represent a potential harvestable resource in Iceland.