New Zealand's 14 deep-water fiords possess persistent salinity stratification and mean estuarine circulation that may serve to isolate populations of marine organisms that have a dispersal larval phase. In order to investigate this idea, we analysed the population structure of the sea star Coscinasterias muricata using a mitochondrial DNA marker. Genetic differentiation among populations of C. muricata was analysed using 366 base pairs of mtDNA D-loop. We compared populations from the fiords with several others sampled from around New Zealand. At a macro-geographical scale (> 1000 km), restricted gene flow between the North and South Islands was observed. At a meso-geographical scale (10–200 km), significant population structure was found among fiords and between fiords and open coast. The pattern of population genetic structure among the fiords suggests a secondary contact between a northern population and a southern one, separated by a contact or mixing zone. These populations may have diverged by the effects of random genetic drift and population isolation as a consequence of the influence of estuarine circulation on dispersal. In northern Fiordland, genetic structure approximated an isolation by distance model. However, the pattern in genetic differences suggests that distance alone cannot explain the most divergent populations and that fiord hydrography may increase the effect of genetic drift within populations in the fiords. Finally, our study indicates that populations within the fiords underwent recent rapid expansion, followed most probably by genetic drift due to a lack of gene flow among the fiords.