Polar bears (Ursus maritimus) are among the Arctic species that are most threatened by climate change. However, little is known about the ecology of polar bears over extended temporal intervals (>50 years). The goal of my thesis is to determine whether historic polar bears from the Lancaster Sound subpopulation underwent ecological shifts in response to a changing Arctic climate throughout the 20th century (1920s-1970s) using stable isotope analysis. Historic polar bears did not undergo significant dietary shifts, remaining apex predators. The consistent reliance on seals and whales as their primary prey indicates continued access to optimal hunting conditions (i.e., sea ice). There is evidence that suggests environmental changes occurred from 1950-1970. This thesis offers novel insights into the historical ecology of polar bears, and provides a long-term isotopic baseline for polar bear ecology that will enable future studies to assess ongoing ecological changes of modern polar bears.