Producing NH₃ via photocatalytic N₂ reduction requires an ideal three-phase boundary (TPB) among N₂ (gas), H₂O (liquid), and a catalyst (solid). A promising strategy for developing TPB system involves photocatalyst passivation with temperature-responsive hydrogels that reversibly switch hydrophilic–hydrophobic characteristics. In this study, a self-controlling TPB system that combines a TiO₂/graphdiyne photocatalyst with poly(N-isopropylacrylamide) (TiO₂/GDY@PNIPAm) is explored. TiO₂/GDY offers excellent solar absorption characteristics, efficient charge separation at the heterojunction, and abundant active sites for N₂ reduction. Owing to a unique photothermal effect, TiO₂/GDY generates a local temperature increase (39.5 °C) under irradiation. The temperature-responsive PNIPAm, utilized as an adaptive porous framework, enables dynamic regulation of interfacial wettability and gas transport within the TPB microenvironment through its hydrophilic–hydrophobic transition, thereby promoting selective N₂ transport while inhibiting H₂O transport. Under solar-light irradiation, the room temperature NH₃ production rate of TiO₂/GDY@PNIPAm (59.5 μmol/gh) exceeds that of TiO₂ (0.46 μmol/gh). These findings provide valuable insights into photocatalyst design and local environment optimization using stimuli-responsive hydrogels toward green NH₃ production.