Island ecosystems provide habitat for many endemic species that may be threatened by nonnative species introductions. We examined nonnative freshwater turtle occurrences and diets to examine potential predation effects on native species in Kawai Nui Marsh, Oahu, Hawaii. No freshwater turtles are native to the Hawaiian Archipelago. The Pond Slider (Trachemys scripta) and Chinese Softshell (Pelodiscus sinensis) were the only turtles found in the marsh after 767 trap days. Trachemys scripta stomachs (n = 50) contained mostly the nonnative plant Commelina diffusa and nonnative snails (Pomacea sp.), whereas Pelodiscus sinensis stomachs (n = 5) contained mostly snails. Interspecific dietary overlap was low and intersexual dietary overlap in the sliders was high, with more diverse female diets. Small, medium, and large size classes of T. scripta stomachs contained different proportions of plant and animal matter, with the small size class containing less plant matter than the medium size class, and the large size class containing a greater volume of animal than plant matter. No native species were found in the stomach contents of the turtles sampled except a freshwater sponge (Heteromyenia baileyi). This lack of native species in their diets may have more to do with the degraded state of the marsh and lack of native taxa than with a preference for nonnative taxa. A potential concern could be nonnative freshwater turtle presence in pristine wetland habitats in Hawaii, because of the higher abundances of native species in those areas.