Abstract: Albacore tuna (Thunnus alalunga) is a top predator in marine ecosystems, yet the relationships among environmental variation, ontogenetic growth, and feeding habits remain insufficiently understood. To characterize its trophic ecology, this study analyzed muscle samples from 192 T. alalunga collected in the South Pacific Ocean and used carbon and nitrogen stable isotope values to examine how trophic position and isotopic niche structure vary with growth, geographic region, and sampling season. The results showed that δ¹³C values ranged from −20.37‰ to −15.50‰ and δ¹⁵N values ranged from 10.52‰ to 21.03‰. The mean trophic position was 4.75 ± 0.71, indicating that albacore tuna occupies a relatively high trophic level in the South Pacific food web. Analysis by fork length group revealed significant positive correlations of fork length with both δ¹³C and δ¹⁵N (P＜0.001), while trophic position also increased with body size and reached its highest value in the 90-100 cm group (5.04 ± 0.71). Isotopic niche width was markedly expanded in the 90-100 cm group. Regional analysis showed that trophic position was highest in the central tropical waters of the south-central Pacific (Region 3 = 5.14 ± 0.61) and lowest in the tropical waters of the southwest Pacific (Region 1 = 4.26 ± 0.79). The temperate waters of the southeastern Pacific Ocean exhibited the widest isotopic niche (SEAc = 9.17), and niche overlap probabilities among regions were generally high. Seasonal analysis indicated no significant differences in trophic position among seasons, whereas the isotopic niche was substantially broader in winter. These findings suggest that the trophic ecology of South Pacific T. alalunga is jointly shaped by ontogenetic growth and spatial heterogeneity. Seasonal variation had a limited effect on trophic position but altered the breadth of resource use. This study provides new evidence for understanding the feeding strategy and ecological adaptation of T. alalunga in the South Pacific Ocean.