<p>Industrial hydrazine (N₂H₄) wastewater is highly toxic and difficult to treat sustainably, and current treatment technologies are typically energy/chemical intensive while conventional photocatalysts either underutilize the solar spectrum or suffer from inefficient charge utilization. Here we induce efficient narrow-bandgap organic photovoltaic catalysts (OPCs) with donor–acceptor heterojunctions that harvest visible to near-infrared solar light and facilitate effective charge separation and transfer to drive remediation of hydrazine wastewater while co-producing hydrogen without external energy input or added sacrificial reagents. Then we effectively enhance the operating stability and performance in complex wastewater matrices by incorporating Al₂O₃-coated OPC nanoparticles. Furthermore, the detailed catalytic mechanism based on proton-coupled electron transfer is revealed through density functional theory calculations combining in situ spectroscopy and isotope experiment. Under simulated sunlight (AM 1.5 G, 100 mW cm⁻²), the optimized OPC nanoparticles reduce 640 ppm N₂H₄ to trace levels (hundredths of ppm) within 5 h, meeting the industrial and agricultural safety standards, with mass/area-normalized hydrogen evolution rates of up to 559.3 ± 28.0 mmol h⁻¹ g⁻¹/117.6 ± 4.7 mmol h⁻¹ m⁻² and good recyclability and no secondary discharge, demonstrating a feasible, efficient and sustainable route for hazardous wastewater remediation.</p>