<p>Understanding the trophic ecology of top marine predators is crucial for assessing ecosystem structure and resilience. We investigated the feeding ecology of the common dolphin (<i>Delphinus delphis</i>) in the East Sea of Korea using stable isotope analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N). Dolphin and prey muscle samples collected between 2020 and 2023 were analyzed to determine trophic position, habitat use, and seasonal dietary variation. δ¹³C values (−18.5‰ to −17.9‰) indicated consistent utilization of mid-shelf pelagic habitats, while significant seasonal shifts in δ¹⁵N values (12.3‰–13.1‰) suggested temporal variations in prey trophic levels. Bayesian mixing models identified Pacific herring (<i>Clupea pallasii</i>) as the dominant prey, contributing up to 56% in summer, whereas sand lance (<i>Ammodytes personatus</i>) and sandfish (<i>Arctoscopus japonicus</i>) were secondary and seasonally restricted prey. Isotopic niche analyses revealed broader trophic flexibility during periods of lower prey abundance (winter–fall) and narrower niches during summer, reflecting opportunistic but seasonally structured feeding behavior. These findings establish an isotopic baseline for <i>D. delphis</i> in Korean waters, highlighting its ecological role as a key mesopredator linking pelagic fish dynamics to higher trophic levels and underscoring the need for continued monitoring under accelerating climatic and anthropogenic changes in the East Sea ecosystem.</p>