ABSTRACT: Dolphins are among the largest and most diverse predators in marine ecosystems, but our understanding of their foraging ecology, which is crucial for ecosystem management, is poor. Delphinus delphis (common dolphins) are found in tropical and temperate waters globally. Stomach content studies indicate they are opportunistic predators that feed locally on abundant small pelagic schooling fish, but information is lacking on how their diet may vary seasonally and with ontogeny. We investigated effects of body length, sex, season, and year on D. delphis diet in the Hauraki Gulf, New Zealand, using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope analysis of 56 skin samples collected during 13 years (2004-2016). Dolphin δ¹⁵N values changed with body length, suggesting ontogenetic dietary variation. Nitrogen isotope values were higher in austral autumn/winter compared to spring/summer, whilst δ¹³C values decreased throughout the study period, suggesting temporal changes in primary productivity likely affecting the dolphins’ diet. Sex had a minor effect on dolphin δ¹³C values, with male and female isotopic niches overlapping by 62.6% and 45.2% (respectively). Our results provide a baseline for detecting future changes in the trophic ecology of D. delphis in a key habitat in New Zealand and highlight their role as a sentinel species in this coastal ecosystem.