The conversion of natural ecosystems into human-modified landscapes (HMLs) is the main driver of biodiversity loss in terrestrial ecosystems, particularly the loss of large predators. Their demise can alter food webs substantially, sometimes releasing smaller carnivores, such as members of the Mustelidae. Nevertheless, even small carnivores must adapt to anthropogenic impacts on food availability, altering their resource use. In this context, the crops grown in agrarian habitats can profoundly affect community assembly. Here, we conducted dietary analysis on 75 Japanese weasel (Mustela itatsi) scats, collected between July 2017 and August 2018, to determine their seasonal food habits in a landscape dominated by rice paddy fields in Saitama prefecture, eastern Japan. From spring to autumn, Japanese weasels consumed predominantly (semi-)aquatic and terrestrial animal taxa, specifically invasive crayfish (Procambarus clarkii), insects (e.g., Coleoptera and Odonata) as well as adult anurans, which are all readily available prey species. In winter, Japanese weasels consumed predominantly fruit (e.g., figs, Ficus carica), with a relative decrease in combined animal content in scats, due to the scarcity of animal prey in dried-out paddy fields and irrigation ditches. Although frugivory is unusual in Mustela species diets, our findings demonstrate that Japanese weasels are capable of adaptive trophic plasticity, enabling them to survive atypical resource conditions in paddy field habitats. To enhance broad efforts to conserve Mustela itatsi in Japan, we recommend the diversification of rice paddy monocultures and encourage winter flooding to increase the availability of aquatic and semi-aquatic animal prey.