The poorly-regulated pet trade is a major source of potential invasive species, with deliberate release often resulting in introductions of non-native species without invasion histories. Predicting the potential impacts of species with no invasion history is particularly difficult. Functional responses (FRs; resource use as a function of resource density) have proven useful in the quantification of ecological impacts of invasive species, and may be used to screen likely impacts of species which lack invasion histories. Here, we used laboratory experiments to quantify the FRs of four freshwater turtles commonly traded as household pets: Trachemys scripta scripta, the yellow-bellied slider; T. s. troostii, the Cumberland slider; Sternotherus odoratus, the common musk turtle; and Kinosternon subrubrum, the Eastern mud turtle, towards representative chironomid and gammarid prey under eight densities in the presence and absence of substrate. All turtles exhibited potentially population destabilising Type II (hyperbolic) FRs towards each prey type, irrespective of the presence of substrate, characterised by high predation rates at low prey densities. Magnitudes of FRs were generally higher for T. s. scripta and T. s. troostii compared to S. odoratus and K. subrubrum. The presence of substrate reduced the magnitude of FRs towards both prey types overall, however, these effects were modest and most pronounced for the two. T. scripta subspecies. We demonstrate marked and sustained predatory impacts of non-native turtles on native prey, irrespective of benthic habitat contexts. We conclude that commonly traded turtles that lack invasion histories could precipitate substantial ecological impact, particularly in freshwater ecosystems where there are no native analogues.