Black marlin (Istiompax indica) are large, highly mobile, apex pelagic predators of tropical and subtropical oceans with high economic importance in western North Pacific fisheries. To assess the ontogenetic trophic ecology of black marlin in the western North Pacific Ocean, the stable isotopic composition of 177 black marlin muscle samples (87–383 cm, eye-orbit fork length (EFL); 33–447 kg, body weight) were examined for ontogenetic shifts and seasonal variability in trophic position. δ¹⁵N and δ¹³C values for black marlin ranged from 9.7 to 14.5‰ and −18.5 to −15.2‰, respectively, and δ¹⁵N values were positively correlated with size. δ¹⁵N and δ¹³C values varied across seasons, with highest δ¹⁵N values in autumn. Based on trophic position estimates and Bayesian mixing models, large black marlin (>250 cm EFL) occupied a higher trophic position (4.8) and fed on more mesopelagic prey (e.g. hairtail, Trichiurus lepturus) compared to smaller individuals, which were estimated to feed more on forage fish and squids as well as hairtail. These results suggest that the trophic position of black marlin varies across life-history stages and seasons, possibly reflecting seasonal movements in surrounding pelagic ecosystems. Trophic level estimates highlight the important ecological role of black marlin as a top predator in pelagic ecosystems, and indicate potential regional importance of mesopelagic fishes in diets.