Ophiura sarsii is a common brittle star species across Arctic and subarctic regions of Atlantic and Pacific oceans. In the Barents Sea O. sarsii is among the dominant echinoderms. We studied genetic diversity of O. sarsii by sequencing the 548 bp fragment of mitochondrial COI gene. O.sarsii demonstrated high genetic diversity in the Barents Sea. Both major Atlantic mtDNA lineages were present in the Barents Sea and were evenly distributed between the northern waters around Svalbard archipelago and the southern part near Murmansk coast of Kola Peninsula. Both regions, as well as other parts of the O.sarsii range, were characterized by high haplotype diversity with a significant number of private haplotypes, being mostly satellites to the two dominant haplotypes, each belonging to a different mtDNA clade. Demographic analyses indicated that the demographic and spatial expansion of Ophiura sarsii in the Barents Sea most plausibly has started during the Bølling–Allerød interstadial, during the deglaciation of the western margin of the Barents Sea.