Fucus vesiculosus L. is one of the most widespread macrophytes in the northwestern Atlantic, ranging from North Carolina (USA) to Greenland (DK). We investigated genetic diversity, population differentiation, patterns of isolation by distance, and putative glacial refugial populations across seven locations from North Carolina (USA) to Cape Breton Island, Nova Scotia (Canada), with microsatellite analyses. Distinct northern versus southern (Delaware–North Carolina) populations were revealed by microsatellite data. Five of six microsatellite loci were fixed in populations in North Carolina, suggesting a recent founder event or a bottleneck, and the same homozygous genotype was found in herbarium materials collected on the North Carolina coast from more than 60 years ago. An additional set of individuals from the northern limit in Greenland was included in our analysis of mitochondrial intergenic spacer (mt IGS) haplotypes in the northwestern Atlantic. Remarkably, 184 of 188 F. vesiculosus specimens from North Carolina to Greenland shared the same haplotype. Recent colonization of the North American shore from Europe is hypothesized based upon the ubiquity of this common haplotype, which was earlier reported from Europe.