Increasing evidence suggests that global warming significantly alters the range and phenology of plants and animals. Whereas thermophilous species usually benefit from rising temperatures, the living conditions of taxa adapted to cooler or continental climates are deteriorating. The woodland ringlet butterfly, Erebia medusa Fabricius (Lepidoptera: Nymphalidae) is one of the continental species that are supposed to be adversely affected by climate change, especially by rising winter temperatures. Here, we conduct an explorative study on the effects of low, moderate, and high winter temperatures on the pre-adult and adult stages of E. medusa in a laboratory experiment. Compared to the two other temperature regimes, the warm winter treatment led to an earlier termination of diapause and higher larval weights at the end of the winter, but significantly lower survival rates. The after-effects of the warm treatment included lower weight of the pupae and adult females, shorter forewings of adult males, and earlier emergence of both adult males and females. In natural environments, which are characterized by a much greater thermal variability and a much higher frequency of soil freeze-thaw events compared to our experiment, the effects of rising winter temperatures might be stronger than in this study. Thus, we conclude that warmer winters pose a non-negligible long-term threat to E. medusa.