Proposed are a photocatalyst, including titanium dioxide particles including titanium dioxide (TiO2), a carbon material located on all or part of the surface of the titanium dioxide particles and including at least one selected from the group consisting of graphene, reduced graphene oxide (rGO), and carbon nanotubes (CNTs), and bimetallic nanoparticles supported on the carbon material and including first metal nanoparticles and second metal nanoparticles, and a water treatment method using the same. In the photocatalyst and the water treatment method using the same, the photocatalyst including bimetallic nanoparticles and graphene oxide is prepared, thereby exhibiting high reduction efficiency and high selectivity to nitrogen gas even without the use of an external electron donor.