SUMMARYParasitism is believed to play an important role in maintaining species diversity, for instance by facilitating coexistence between competing host species. However, the possibility that environmental factors may govern the outcome of parasite-mediated competition has rarely been considered. The closely related amphipodsCorophium volutatorandCorophium arenariumboth serve as second intermediate host for detrimental trematodes.Corophium volutatoris the superior competitor of the two, but also suffers from higher mortality when exposed to infective trematode stages. Here, we report parasite-mediated competitive release ofC. arenariumin an intertidal habitat, in part triggered by unusually high temperatures linked to the North Atlantic climate oscillation (NAO). The elevated temperatures accelerated the transmission of cercariae from sympatric first intermediate hosts (mud snails) to amphipods, causing a local collapse of the parasite-sensitiveC. volutatorpopulation and concordant increase in the abundance of the competitively inferiorC. arenarium.