Studies using stable-isotope analyses of feathers to determine molt locations in marine birds are increasingly common but generally lack verification through ground-truthing. In this study, we examined the stable isotope compositions of wing feathers from King Eiders (Somateria spectabilis) implanted with satellite-transmitters. We compared stable carbon, nitrogen, and hydrogen isotope compositions (δ¹³C, δ¹⁵N, and δD, respectively) of primary feathers with the geographic location of wing molt in the subsequent year. Longitude of molt locations of eiders was highly correlated with δ¹³C (r² = 0.69, N = 12) and δD (r² = 0.90, N = 12) isotope values from their primary feathers grown in the previous year. There was no relationship between δ¹⁵N and location of wing molt (r² = 0.007, N = 12). The results of this study provide further evidence for site fidelity during wing molt by King Eiders.