Although the Oriental Magpie Pica serica does not occur naturally in Japan, a small population has been established in the urban and suburban areas of Tomakomai City, southern Hokkaido since the 1990s. Here, we estimated the proportion of pet food in the magpie diet by using stable isotope analysis to evaluate its contribution to the successful establishment of the population. For tissue analysis, we collected magpie eggshell membranes and feathers, which can indicate the magpie diet during the egg-laying period (spring) and the moulting season (summer to early autumn), respectively. We collected ground-dwelling invertebrates from the field and several commercial pet food varieties as putative food sources. Pet food was isotopically distinct from invertebrates in both δ¹³C (–19.7 ‰ vs. –27.3 ‰, respectively) and δ¹⁵N (4.0 ‰ and 2.6 ‰, respectively). Our stable isotope analysis with a Bayesian mixed model framework, MixSIAR, showed that the proportion of pet food in the magpie diet was approximately 44.6% and 21.8% based on the eggshell membrane and feather samples, respectively. These estimates imply that the Oriental Magpie has succeeded in establishing a population in Tomakomai by exploiting pet food.