Although the Haber–Bosch process supports the growth of modern agriculture with abundant ammonia and fertilizer production, substantial energy consumption and enormous greenhouse emissions demand an alternative and sustainable approach. Here, we report a novel approach that combines the non-photosynthetic bacterium Shewanella oneidensis MR-1 (S. oneidensis MR-1) with cadmium sulfide (CdS) nanoparticles (NPs) to enable the photosynthesis of ammonium (NH₄⁺) from nitrate (NO₃⁻) using photoexcited electrons as donors. The NO₃⁻ reduction efficiency reached almost 100%, with an NH₄⁺ production selectivity of over 90%. The maximum instantaneous quantum efficiency was 3.01% under light irradiation. The reverse metal-reducing (Mtr) pathway is responsible for the transfer of photoexcited electrons to intracellular compartments. Parallel reaction monitoring analysis illustrated that NO₃⁻ to NH₄⁺ was produced via the dissimilatory nitrate reduction to ammonium (DNRA) pathway in S. oneidensis MR-1. This study provides a facile strategy for light-driven ambient NH₄⁺ synthesis and solar-to-chemical conversion.