A main molecular subdivision in the circumpolar Macoma balthica complex has been described between Atlantic and Pacific taxa. In NE Europe, the clams of the White and Barents Seas, however, show deviant genetic structures. Using allozyme and mitochondrial DNA data, we explore the hypothesis that these deviations result from hybridization between an Atlantic (M. b. rubra) and an invading Pacific (M. b. balthica) lineage. A practically pure Atlantic Macoma extends from France north to the Varanger Peninsula (NE Norway), whereas populations farther east have genetic compositions intermediate between true Atlantic and true Pacific. Admixture estimates range from 32 to 90% Pacific contribution, with a notable deviation in a nearly pure Atlantic outpost in the Mezen Bay (NE White Sea). The pattern of variation is not one of a simple collinear mixing however. Different characters exhibit different degrees of introgression, and the relative introgression varies regionally. Yet, there are practically no interlocus genotypic disequilibria between the diverged loci, which brings out the White Sea-Barents Sea M. balthica as the best-documented marine animal hybrid swarms so far, arisen through amalgamation of genomes previously isolated since pre-Pleistocene times. On top of the main admixture pattern, strong geographical structuring is also seen in characters unrelated to the principal systematic distinction. The persistence of the regional patterns indicates restricted gene flow at the present time, despite the high dispersal potential of the species. The causes of this structuring could be in a complex history of colonization events and features of local hydrography enhancing isolation and divergence of populations.