I examined living ostracode assemblages from the northern Bering Sea, collected between 1976 to 2010, and from the Chukchi Sea, collected in 2009 and 2010, to determine how climatic and oceanographic changes affect ostracode species distributions. I found the Bering Sea assemblage to be transitional in species composition between those inhabiting western Arctic continental shelves and the subarctic Gulf of Alaska. Temporal changes in the Bering Sea assemblage provide evidence that decadal temperature changes have affected species composition. For example, the proportion of Normanicythere leioderma, a predominantly Arctic species, decreased from 70% of the total assemblage population in 1999 to 15% by 2006. This decrease coincided with a shift in the Arctic Oscillation toward a positive mode and warmer Bering sea-surface temperatures beginning in the early 2000s. My results support the hypothesis that recent ocean temperature changes in the Bering-Chukchi Sea region are changing species composition in benthic ecosystems.