The Svalbard archipelago is home to an endemic wild reindeer subspecies that is uniquely suited to its arctic environment. Following colonization just 6000 years ago, this population rapidly evolved numerous adaptive traits. ColdRein takes advantage of this uncommon opportunity, a natural experiment that has already taken place, to directly assess how, and how quickly, an isolated animal population was able to adapt to a harsh and highly stochastic environment. Using abundant, readily available samples from both present-day and ancient reindeer, we will generate a large circumpolar population genomic dataset (including an extinct subspecies from Greenland), and then apply powerful evolutionary simulations and genomic scanning to determine how long-term fluctuations in population size and dispersal reflect environmental dynamics, and to search the Svalbard reindeer’s genome for signatures of positive selection. Finally, we’ll look directly back in time through the history of Svalbard reindeer evolution, using palaeogenomics to track adaptive alleles’ origin and their rise in frequency. In doing so, ColdRein will generate fundamental insights into the process of rapid environmental adaptation in a large mammal.