Sabine's gull treated as comprising a monotypic genus; it is only placed within the genus "Larus" when the genus is enlarged. The black bill and notched tail are almost unique within the gulls, as they are shared only with the swallow-tailed gull of the Galapagos. On the basis of this the two species were often thought to be each other's closest relatives, a hypothesis ruled out by a number of behaviour and ecological differences. Mitochondrial DNA studies confirmed that they are not closely related, and the closest relative of the Sabine's gull is now thought to be the ivory gull, another Arctic
Sabine's gull species. The two species are thought to have separated around 2 million years ago, longer ago than most groups of gull species. Geographical variation is slight; birds from Alaska are slightly darker and perhaps bigger. Most authorities recognise no races, but a few recognise four based on size and mantle (back) colour. The Handbook of the Birds of the World recognises four subspecies. The nominate subspecies, "X. s. sabini", breeds from the Canadian Arctic to Greenland. "X. s. palaearctica" (Stegman, 1934) breeds from Spitsbergen to the Taymyr Peninsula in Russia, and "X. s. tschuktschorum" (Portenko, 1939) breeds on the Chukotskiy