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Climate-mediated sea ice declines have led to alterations in ecosystem composition, phenology, and primary productivity, potentially altering trophic dynamics in Arctic marine ecosystems. As one of the Arctic's top predators and a species dependent on sea ice habitat for survival, polar bears Ursus maritimus are at risk of habitat loss from sea ice declines, and therefore have been used to monitor the effects of climate change in the Arctic. We used stable isotope ( 15 N, 13 C) values of 806 hair samples collected from Western Hudson Bay (WH) polar bears from 1993-1994 and 2004-2016 to examine variations in isotopic niche size and diet within the population, patterns in foraging ecology over time, and the relationship between hair isotopic values and sea ice dynamics. We found significant variation in isotopic values between age-and sex-classes. Adult males had the highest 15 N and 13 C values and the largest isotopic niches, reflecting broader diets. In contrast, adult females (with and without cubs) and subadults differed isotopically from adult males, and their relatively smaller isotopic niches potentially make them more vulnerable to habitat change. Population 15 N values significantly increased over time from 1993 (mean SD: 18.8 0.5 ) to 2016 (19.5 0.7 ), while 13 C values significantly decreased from 1993 (-16.2 0.3 ) until 2011 (-17.7 0.2 ) before increasing until 2016 (-17.0 0.3 ). Values of 13 C were significantly correlated with length of the open-water period, suggesting that this apex predator's foraging ecology was affected by climate change. We suggest that WH polar bears may have undergone a significant dietary niche shift and/or that the baseline isotopic values in this ecosystem may have changed over the last 25 yr in response to climate change.