<p>The synthesis of high-performance photocathodes capable of converting seawater into clean hydrogen fuel represents a significant step toward advancing renewable energy technologies. In this work, we report the synthesis and evaluation of a novel nanosheet-based composite consisting of tungsten(VI) oxide sulfide and poly(<i>o</i>-aminothiophenol) (WO_3−<i>X</i>S_<i>X</i>/POATP). The nanosheets, with an average thickness of ~ 20 nm and a crystallite size of ~ 15 nm, exhibit uniform morphology and well-defined surface roughness, features that enhance light harvesting and facilitate charge transport. Optical studies indicate that the composite possesses a broad absorption range in the visible region with an estimated bandgap of 1.7 eV, making it suitable for solar-driven photocatalytic processes. The photocathode’s hydrogen evolution performance was tested using both natural seawater and an artificial seawater analogue. Under simulated sunlight, the hydrogen production rates reached 1.8 µmol h⁻¹ cm⁻² for natural seawater and 0.4 µmol h⁻¹ cm⁻² for artificial seawater, highlighting its strong capability for operation under real environmental conditions. Further analysis of the photocurrent density (<i>J</i>_ph) at −0.95 V under different photon energies revealed an increase from −0.028 mA cm⁻² at 1.7 eV to a maximum of −0.035 mA cm⁻² at 2.8 eV, followed by a slight rise to −0.036 mA cm⁻² at 3.6 eV. These variations confirm the high photoresponsiveness and sensitivity of the system to different light inputs. In addition to its excellent performance, the composite offers practical advantages such as the use of cost-effective materials, straightforward fabrication methods, and environmental compatibility. These attributes make the WO_3−<i>X</i>S_<i>X</i>/POATP nanosheet photocathode a strong candidate for scalable, sustainable hydrogen production directly from seawater, supporting global initiatives aimed at reducing reliance on fossil fuels and promoting clean energy adoption.</p>