Admixture, the mixing between divergent genomes, is widely thought to hinder local adaptation. However, a handful of recent studies have suggested that admixture can promote local adaptation. The Soay sheep of St Kilda are a primitive breed that have been the subject of a well documented long-term study, where data on life history, morphology, parasite burden, and pedigree information have been collected for over 7000 sheep. We followed up historical anecdotal evidence of an admixture event in the mid-late 1800s by screening 486 Soay sheep on a 50k ovine SNP chip. We found evidence for such a recent admixture event with a more modern, domesticated breed in the analysed genomic data. Utilising the HapMap dataset of over 60 different sheep breeds we showed that several haplotypes, previously demonstrated to be under selection in this population, have been introduced into Soay sheep from more modern breeds. Our study demonstrates that the introgression of domesticated alleles into wild populations is not necessarily disadvantageous and in fact it may provide a novel source of genetic variation capable of generating rapid evolutionary changes.