Microplastics and nanoplastics are emerging contaminants in aquatic and terrestrial environments. Due to the challenges in detecting these small plastics in complex soil matrices, data about their abundance in terrestrial environments are scarce and our knowledge of their impacts on terrestrial organisms is severely lacking. Here, we introduced a well-established terrestrial model organism – the fruit fly (Drosophila melanogaster) – to study the chronic adverse effects of model micro- and nanoplastics. The toxicity of model dialyzed polystyrene spheres (1 μm and 20 nm) was assessed via dietary exposures to a wide range of concentrations (0.01 to 100 ppm). In a first experiment, flies were exposed from larval to adult stage for 13 days. Uptake of both particle sizes in the gastrointestinal tract of larvae was observed, with depuration times of 1 and 24 h for 20 nm and 1 μm particles, respectively. In adults, only 1 μm particles accumulated to detectable levels and nano computed tomography imaging revealed intestinal damage. Both micro- and nanoplastics significantly affected locomotion while mortality, development, and fertility were not significantly affected. A second 8 day experiment focused on the daily behavior of adults exposed to micro- and nanoplastics. We observed no effect on circadian rhythms but an increase in daily activity after micro- and nanoplastic exposure at 50 ppm, likely in response to a reduction in nutrient absorption due to the change in diet or intestinal damage. Overall, dietary exposures to clean spherical polystyrene micro- and nanoplastics caused low toxicity but significant sublethal effects in the fruit fly. This study establishes a baseline understanding of the impacts of model micro- and nanoplastic exposure to the fruit fly and motivates the need for further work focusing on naturally weathered plastic debris.