Abstract: Climate change is likely to have a profound impact on the marine intertidal, where ecosystem resilience already suffers from additional human stressors. Its impact largely depends on how the key species of this ecosystem respond to increasing temperature. Basically, they are confined to three possible strategies: to (1) move, (2) be plastic or (3) evolve. Our overall objective was to uncover responses of the habitat provisioning key species Fucus serratus that have the potential to provoke changes throughout the entire North Atlantic intertidal ecosystem. More specifically, we aimed to (1) predict distributional changes of this macroalgae under three IPCC scenarios with ecological niche models using the program Maxent and (2) identify genetic changes and putatively selected loci in four populations that we had sampled at the beginning and end of the previous decade and genotyped for 31 microsatellite loci. Our niche models predict that the fundamental niche of F. serratus shifts up to 1000 km north within the coming two centuries. At the southern limit of distribution, in Spain, the allelic richness had already significantly declined by 14% over the past decade. Although each of the four genotyped populations revealed significant genetic changes over the past decade, outlier loci coincided only partly between them. If the southern populations are despite high genetic diversity unable to evolve tolerance to increasing temperature, they will become extinct under the predicted northward niche shift. The presence of outlier loci however suggests adaptive responses of F. serratus to climate change that might mitigate the predicted extent of northward retreat. Otherwise, the affected coastal regions may undergo substantial ecosystem restructuring with an unpredictable impact on human society and economy.