We analysed 229 Eurasian otters from eastern and north-western Germany, Denmark and southern Sweden based on sequences of the mtDNA control region and genotypes at 12 polymorphic nuclear microsatellite loci. The main focus of the study lay on the north-west German otters from Schleswig-Holstein, which represent a newly established and expanding population recolonizing a formerly inhabited region, thereby closing the distributional gap between the large eastern population and the long-isolated and genetically depleted Danish population. As found before in this species, mtDNA variability was very low but we identified a hitherto unknown haplotype in Sweden. Expected heterozygosities were between 0.46 and 0.69 and thus within the range known from the literature. There were only weak signs of a founder effect in the Schleswig-Holstein otters with respect to allelic diversity (but not heterozygosity). Nevertheless, there was a statistically significant bottleneck signal based on deviations from mutation-drift equilibrium in Schleswig-Holstein. All statistical approaches (amovas, factorial correspondence analysis, assignment tests and Bayesian structure analysis) unequivocally showed that north-western Germany has so far been predominantly recolonized by otters of east German origin, but we also found evidence of admixture through immigration from Denmark. Both the Danish and the north-west German otter population will benefit from further exchange in the wake of the ongoing range expansion.