Most conventional photoelectrochemical-based methods for synthesizing NH3 show low selectivity due to the generation of H-2 as a by-product. In principle, two types of reaction mechanisms can occur in the reduction of N-2 to NH3. One is an associative pathway in which N-2 molecules on the catalyst are hydrogenated. The other is a dissociative pathway in which nitrogen and hydrogen react after the cleavage of the strong N-2 triple bond. Understanding the mechanism of NH3 formation on the electrode will facilitate the development of selective and efficient NH3 synthesis techniques. In this study, we constructed a two-electrode system composed of a strontium titanate photocatalytic anode in which the plasmon effect is expressed by plasmonic gold nanoparticles and a zirconium cathode, which was connected to the external circuit to investigate the reaction by electrochemical analysis in addition to analysis of the product. The bias and pH dependences of the reaction were then systematically investigated, and the plasmon-induced synthesis of NH3 on Zr was proposed to proceed via an associative pathway.