Knowing how migratory pelagic predators use food web resources during early life stages is essential for understanding population dynamics and recruitment. Feeding habits of ~3 to 4 mo old Atlantic bluefin tuna (ABFT) Thunnus thynnus from 2 Mediterranean nearshore areas (Cartagena and Cambrils, Spain) showing distinct oceanographic features were investigated using stomach-content (SCA) and stable-isotope (SIA) analyses. SCA showed diet variability between areas and years; further correspondence analysis revealed that epipelagic fishes (primarily anchovy and sardine) constitute the staple diet of age-0 ABFT in Cambrils, whereas mesopelagic fishes (especially myctophids) and squid were consumed in significant amounts by ABFT in Cartagena. δ13C values differed significantly between tissues (white muscle and liver), areas and years (2009 and 2010), while δ15N values were not significantly different. In all cases, mean δ13C values were significantly higher in muscle than in liver, while differences in δ13C between locations and years were also observed in some instances. Estimations of trophic niche width using the antilogarithm of Shannon’s entropy, exp(H), and from stable isotope Bayesian ellipses, SEAc, indicated that niche width was generally larger in samples from Cambrils than in those from Cartagena. Moreover, SEAc analysis suggested that trophic niches were broader in 2010 than in 2009. Our results indicate a low degree of feeding specialisation and suggest that, like older classes, age-0 ABFT rely on a wide prey range, easily adapting their foraging behaviour to prey availability and distribution in the habitat. This research assesses the potential of using stable isotope techniques combined with SCA for the characterization of spatio-temporal variability in the diet of juvenile top predators.