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Simple Summary: Most of the ecosystems around the world are experiencing high levels of stress. Invasive species are one of the major threats to biodiversity. To successfully prevent and manage this conservation issue, a good understanding of ecological and biological processes involving the native and introduced species is required. This is often challenging due to the interaction of many stressors within an ecosystem, hindering direct changes caused by the invader. Here, we present a unique situation where an invading snow crab is the only disturbing factor in the benthic ecosystem of an arctic fjord. This study looks at how the crabs' feeding habits change as the invaded ecosystem degrades due to their foraging. Stomach content and stable isotope analyses have revealed that while the prey items have changed, and crabs are forced to feed on less accessible and less nutritional food, their trophic position has not altered. Changes in feeding habits have occurred within the same trophic level. Following the depletion of most nutritional prey items, the number of crabs in the bay has collapsed. We expect a further decrease in crab numbers, a possible partial restoration of the benthic community, and a repeat of the cycle after new settlements of young crabs. The introduction of a new species can lead to substantial changes in a new ecosystem. Local and introduced species' survival depends on their ability to adapt to the new environment. Studying such adaptations is often hindered by multiple factors affecting the ecosystem. The introduction of a large predatory snow crab, Chionoecetes opilio, into the Kara Sea, is a unique invasive species affecting an otherwise undisturbed ecosystem. The crab has caused drastic changes in the macro- and megabenthic taxonomic structure, abundance, and biomass of the most common species in an Arctic fjord, Blagopoluchiya Bay. Stomach content and stable isotope analysis were applied to study crabs' feeding habits. As the abundance of the most common prey items diminished, the crabs switched to other less accessible food. Prior to substantial changes in benthic communities, the diet of the snow crabs was similar to that of other invaded and native areas, where animal food predominates. However, with the degradation of the ecosystem, detritus contribution has substantially increased. The changes in prey items did not change the crab's trophic status, and they continued to feed within the same trophic niche. In the depleted benthic communities of Blagopoluchiya Bay, the snow crab is forced to use all available food sources. [ABSTRACT FROM AUTHOR]