Temperatures are rising at an unprecedented rate in the Arctic, and affecting the sensitive hydrological cycle with increased freshwater inputs to the marine environment. A deep-time analogue often used to understand and predict the environmental and ecological impacts of global warming is the early–middle Eocene greenhouse (ca. 53–38 Ma). Although the terrestrial Arctic during this interval is well documented, marine records are rare and largely based on one central Arctic Ocean site at Lomonosov Ridge. Here we estimate a mean paleosalinity of 12.7 PSU for coastal waters of the western Arctic Ocean by incorporating δ18OCO3 values of shark teeth with estimates of Eocene Arctic paleotemperature and freshwater δ18O values into a salinity model. This paleosalinity is substantially lower than estimates by others for the Eocene central Arctic Ocean (21–25 PSU) and today’s Arctic Ocean (32–35 PSU). Our results provide the first Eocene salinity estimate for the western Arctic Ocean and, when considered with prior results from Lomonosov Ridge, suggest a reduced surface salinity, but larger salinity gradient (∼10 PSU) across the Eocene Arctic Ocean during the early–middle Eocene greenhouse. From an ecologic perspective, while today’s lamniform sharks are largely intolerant of low salinity, Eocene species were abundant in the brackish Arctic Ocean, suggesting that past environmental tolerances were much greater than today.