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The rapid warming of the Arctic may not only alter species' abundances and distributions, but likely also the trophic interactions within and between ecosystems. On the high-arctic tundra, extreme warm spells and associated rain-on-snow events in winter can encapsulate the vegetation entirely in ground-ice (i.e., basal ice) and directly or indirectly affect plants, herbivores, and carnivores. However, the implications of such extreme events for trophic interactions and food-web ecology are generally far from understood. Here, we show that wild Svalbard reindeer populations increasingly isolated by lack of sea-ice respond to rain-on-snow and ice-locked pastures by increased kelp consumption. Based on annual population surveys in late winters 2006-2015, the proportion of individual reindeer feeding along the shoreline increased the icier the winter. Stable isotope values (delta S-34, delta C-13, delta N-15) of plants, washed-ashore kelp, and fresh reindeer feces collected along coast-inland gradients, confirmed ingestion of marine biomass by the reindeer in the shoreline habitat. Thus, even on remote islands and peninsulas increasingly isolated by sea-ice loss, effects of climate change may be buffered in part by behavioral plasticity and increased use of resource subsidies. This marine dimension of a terrestrial herbivore's realized foraging niche adds to evidence that global warming significantly alters trophic interactions as well as meta-ecosystem processes.