Aim: To investigate biogeographic origins and admixture among pan-Arctic Cassiope tetragona populations. Location: Circumpolar Arctic and Coastal British Columbia alpine. Taxon: Cassiope tetragona L., subspecies saximontana and tetragona, outgroup C. mertensiana. Methods: We built genotyping-by-sequencing libraries using Cassiope tetragona leaf tissue collections from 36 Arctic locations, including two populations from ~200-500-year-old plants from under glacial ice on Ellesmere Island, Canada. C. mertensiana was collected as an outgroup from two locations in British Columbia, Canada. Results: Pan-Arctic nucleotide diversity was positively correlated with average summer temperature and timing of Holocene deglaciation. Variants from non-local genetic clusters were more frequent in present-day versus historic plants on Ellesmere Island. Arctic populations displayed an excess of heterozygosity compared to Hardy Weinberg expectations. In samples from Kluane, Yukon, (C. tetragona ssp. tetragona), the proportion of C. tetragona ssp. saximontana ancestry (found at lower latitudes) was greater at lower elevations. Conclusions: The positive correlation of genetic diversity with temperature implies that populations at warmer locations have a larger effective population size, possibly due to greater/longer gene flow at these sites. The pattern of increasing admixture with decreasing elevation in Kluane suggests divergent selection for two subspecies of C. tetragona in different environments. On Ellesmere Island, gene flow appears to have been limited in the past compared to present day, consistent with warmer climates increasing diversity. Identified genetic clusters likely derive from separate refugia during the Pleistocene (Russia, British Columbia, and Beringia) but still share some gene flow. The higher-than-expected heterozygosity indicates outbreeding due to long-distance dispersal and possible polyploidy in two local populations.