The red-eared slider, Trachemys scripta elegans, is a freshwater turtle native to the South Central United States of America and Northeastern Mexico. It was introduced into Japan in the late 1960s and is currently the dominant and widely distributed species throughout the country. The species is salt tolerant, and its distribution has recently expanded into tidal flats; however, its ecology in the tidal flats is unknown. The aim of the present study was to analyze the diet of T. s. elegans inhabiting the tidal flats. We collected 46 individuals from the tidal flats and adjacent freshwater waterway of the Tanakagawa River in the Mie Prefecture in November 2014 and from April to November 2015. We compared the intestinal content of the turtles from both the habitats. They consumed a wide variety of plants and animals in both the habitats, with no difference between the habitats in terms of food intake per body weight. There was a higher ratio of animal to plant matter in juveniles and adults from the tidal flats than that in those from the freshwater waterways. In this animal matter, there was a high incidence of decapod species, such as the tidal flat crabs Helice tridens and Macrophthalmus japonicus. To the best of our knowledge, the present study is the first to suggest that the red-eared slider affects the tidal flat ecosystems by preying on the native tidal flat organisms.