Mitochondrial introgression from Bos to Bison is predicted from three premises. (1) Extant European bison have Bos-like mitochondria whereas American bison are more Yak-like, (2) Female F1 from crosses of modern Bos x Bison are fertile, and (3) Bos primigenius and Bison spp. of Pleistocene Europe had considerable geographical and temporal overlap. Using mitochondrial DNA from 64 ancient bison, we identified a previously uncharacterised bison genetic clade also containing Bos-like mitochondria. Nuclear SNP analyses suggest the Bos introgression is shared by both modern European bison and the extinct clade. A dated mitochondrial tree provides a minimum age for Bos introgression of ~120 kya. We suggest that hybridisation, or at least outbreeding, may have been a common characteristic of bovinae in Pleistocene Europe in response to range contractions and expansions triggered by the Quaternary warming and cooling cycles. Hybridisation may have reduced inbreeding depression during range contractions, whereas wavefronts of population expansions likely radiated through many existing populations, providing genetic heterogeneity for fast adaptation to local conditions.