Abstract <p>The nitrogen and carbon isotopic signatures (δ¹³C and δ¹⁵N values) were analyzed in the skin of gray whales (<i>Eschrichtius robustus</i>) from the western and Chukotka–California (eastern) populations. Samples were collected in three feeding grounds of gray whales—off the coasts of Sakhalin Island (Piltun feeding ground), Kamchatka (Kamchatka feeding ground), and Chukotka (Chukchi feeding ground)—during the summer feeding period. The resulting data demonstrate changes in the isotopic composition of western gray whales during their migration from the Kamchatka feeding ground to the Piltun feeding ground. The isotopic niches of gray whales from different feeding grounds, including their similarities, differences, and niche overlap, are described. The content of the stable nitrogen isotope (¹⁵N) is significantly lower in whales of the western population than in whales of the Chukotka–California population. No differences in δ¹³C and δ¹⁵N values have been detected between whales of different sex and age groups. In whales feeding off the coast of Chukotka, δ¹³C and δ¹⁵N values were negatively correlated, whereas off Sakhalin a positive correlation was observed, and off Kamchatka no correlation was detected. The isotopic signatures of whales from different feeding grounds exhibited distinct and unique characteristics.</p>