<p>This study revealed the previously unappreciated consumption of terrestrial plant matter by Ryukyu-ayu <i>Plecoglossus altivelis ryukyuensis</i>, enabled by <i>psbA</i>-based DNA metabarcoding. Although ferns are a minor dietary component, their presence indicates the facultative consumption of terrestrial macrophytes. In terms of the origin of this material, we hypothesize that they ingest drifting material. Eustigmatophytes, a dominant dietary element, have not been previously reported in the diets of ayu <i>Plecoglossus altivelis altivelis</i>. Their presence, along with that of terrestrial ferns, suggests a terrestrial origin. Individual Ryukyu-ayu exhibit a dietary dichotomy: some primarily consume eustigmatophytes, ferns, and seed plants (drift-feeding), while others consume diatoms (grazing). This shift may be correlated to resource availability and competition. Downstream sites with reduced algal productivity and competition from grazing gobies favor drift-feeding in Ryukyu-ayu. A positive correlation existed between dietary diversity and individual conditions in Ryukyu-ayu, indicating that diverse diets would promote better growth. Drift-feeding represents an adaptive foraging tactic within the island’s unique ecosystem, characterized by low riverine algal productivity and frequent rainfall, leading to a substantial influx of terrestrial particulate organic matter. In conclusion, Ryukyu-ayu exhibits dietary flexibility and adaptation to the dynamic island river food web. This study highlights the importance of diverse food resources and interspecific relationships, such as the Amami rabbit-Ryukyu-ayu connection, for effective Ryukyu-ayu conservation.</p>