This dataset includes carbon and nitrogen isotope concentrations measured in polar bear hair and marine mammal prey muscle and blubber samples collected 1978-2019 in the Beaufort and Chukchi Seas. Marine mammal prey samples were collected opportunistically either from polar bear seal kill sites or from marine mammals harvested by Native hunters, including ringed seal, bearded seal, gray whale, Pacific walrus, bowhead whale, beluga whale, spotted seal and ribbon seal. All prey samples were collected within the Alaska Beaufort or Chukchi Sea with the exception of gray whale samples that were collected in Chukotka. More specific information on the ___location in which prey samples were collected are provided within the data. Hair was collected from polar bears captured on the sea ice or land in the spring or fall from 1987-1989 and 2008-2017 in the Russia and/or Alaska portions of the Chukchi Sea and 1983-1989 and 2004-2016 in the Alaska Beaufort Sea. Whole hairs were analyzed for carbon and nitrogen isotopes. Prey samples were analyzed separately as blubber lipids, lipid-extracted blubber protein, bulk muscle, and lipid-extracted muscle. Not all tissues were available for all individual prey samples.