Stomach contents of 437 age-0 year Pacific bluefin tuna Thunnus orientalis (20·3–59·4 cm fork length, LF) caught in the Tsushima Current and the Kuroshio regions around Japan were examined to investigate their ontogenetic diet shift. Prey compositions were diverse and different between regions. Although the seasonal growth patterns were different between regions, ontogenetic diet shifts shared a common pattern. In the Tsushima Current region (Sea of Japan), small T. orientalis (20–25 cm LF) preyed upon small squid (juvenile Enoploteuthis chunii), and larger ones (25–35 cm LF) gradually shifted their diet to mesopelagic fish (Maurolicus japonicus). In the Kuroshio region (Pacific Ocean), small T. orientalis (20–25 cm LF) preyed upon small zooplankton (mostly crustacean larvae), and larger ones (25–40 cm LF) shifted to epipelagic fishes (Etrumeus teres, Sardinops melanostictus and Engraulis japonicus). The observed data suggest that T. orientalis switch to a diet more based on fish prey items, which have more body mass and greater swimming ability than small squid and zooplankton, after they reach a LF of 25 cm.