The omnivorous, red-eared slider (RES) turtle, Trachemys scripta elegans (Emydidae), is an alien species introduced to Japan from North America. Young RES mainly eat animal matter, but shift to a diet that includes plants as they grow. In recent years, feeding damage by RES to crops, other cultivated plants and endangered plants has been reported in Japan. Few case studies, however, have focused on the feeding damage by this species to Japanese wild plants. Our study therefore aimed to quantify the feeding damage to Japanese wild plants by RES by comparing data on plants available in their habitat to the stomach contents of the turtles. From April to December, 2018, and April to November, 2019, we captured RES at three ponds in Okayama Prefecture, Japan, during two days each month. A total of 39 individual turtles (14 males and 25 females) were captured using basket traps in the ponds. A total of 35 families, 60 genera and 74 species of vascular plants and algae were found in the ponds. The stomach contents of adult RES from the three ponds contained more plant matter, including algae (71%), than animal matter (29%), confirming previous reports. The stomach contents also included Japanese native plants (Tamasu pond: 63%, Narazu pond: 40%, Sayama pond: 57%, with the total contents of all three ponds being 56%). The findings indicate that Japanese native plant species are included in the diet of the RES.