This dataset includes carbon and nitrogen isotope concentrations measured in polar bear hair and marine mammal prey samples collected 1978-2017 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. 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 Alaskan portions of the Chukchi Sea and 1983-1989 and 2004-2016 in the Alaskan Beaufort Sea. Whole hairs were analyzed for carbon and nitrogen isotopes. Prey samples were analyzed separately as blubber lipids, blubber protein (with lipids extracted), bulk muscle, and lipid-extracted muscle. Not all tissues were available for all individual prey samples.