Detailed knowledge of marine mammal diet is critical to understand their ecological roles and for the adequate management of marine resources. Antarctic fur seals ( Arctocephalus gazella) in the Southern Atlantic Ocean rely largely on Antarctic krill ( Euphausia superba) during the summer months, but their winter diet remains largely unknown. Here, stable carbon and nitrogen isotope ratios in whiskers and blood of young (2–3 years old) and subadult (4–7 years old) Antarctic fur seal males from the South Shetland Islands were used to assess the seasonal and interannual changes in the krill contribution to their diet. The stable isotope ratios revealed that krill dominated the diet of young and subadult male Antarctic fur seals year-round, with penguins, and not fish, as the second major prey, regardless of year and season. The year-round reliance of young and subadult male Antarctic fur seals on krill should be considered for ecosystem-based fisheries management, as they represent the bulk of the fur seal population remaining off Antarctica during the winter months.