Climate change enables expansion of southern species whereas many boreal species retreat northwards. Range shifts create novel interactions that can affect abundance and distribution via competition and/or hybridization. It is possible that the mountain hare (Lepus timidus) long-term decline in northern Europe is linked to competition with the brown hare (L. europaeus). In order to understand consequences of hare interactions at high latitudes, we determined geographical extent of hybridization using mitochondrial DNA (mtDNA) introgression as a proxy. Additionally, we compared species distribution and snow conditions in Finland with introgression frequency. We found that introgression is bidirectional but direction is mostly from the mountain hare to the brown hare. Approximately 10% of brown hares have introgressed mtDNA, being most common at the northern expansion front of their distribution range. Introgression is likely linked to brown hare expansion facilitated by climate change, but due to complex nature of evolutionary processes, selective advantage cannot be fully excluded.