Pelagic mesopredators are abundant in many marine ecosystems and exert strong top-down influence on food webs. We explored the dietary niche of Pomatomus saltatrix in eastern Australia, using a classification tree analysis to identify key factors driving diet variation. P. saltatrix was shown to be an opportunistic generalist predator which exhibited increased baitfish consumption, and decreased crustacean consumption, with increasing size. The classification tree analysis showed that body size and latitude had the greatest influence on the diet of P. saltatrix, with significant ontogenetic diet shifts occurring at 8 and 30 cm fork length (FL). While piscivory is evident in the majority of P. saltatrix diets by ~8 cm FL, crustaceans are almost entirely absent from the diet after ~30 cm FL. The importance of latitude was likely related to the broad-scale oceanography in the study region, including the East Australian Current and its separation from the continental shelf. The classification tree analysis is a powerful framework for identifying important variables in diet composition.