Engineering of defects in semiconductors provides an effective protocol for improving photocatalytic N₂ conversion efficiency. This review focuses on the state-of-the-art progress in defect engineering of photocatalysts for the N₂ reduction toward ammonia. The basic principles and mechanisms of thermal catalyzed and photon-induced N₂ reduction are first concisely recapped, including relevant properties of the N₂ molecule, reaction pathways, and NH₃ quantification methods. Subsequently, defect classification, synthesis strategies, and identification techniques are compendiously summarized. Advances of in situ characterization techniques for monitoring defect state during the N₂ reduction process are also described. Especially, various surface defect strategies and their critical roles in improving the N₂ photoreduction performance are highlighted, including surface vacancies (i.e., anionic vacancies and cationic vacancies), heteroatom doping (i.e., metal element doping and nonmetal element doping), and atomically defined surface sites. Finally, future opportunities and challenges as well as perspectives on further development of defect-engineered photocatalysts for the nitrogen reduction to ammonia are presented. It is expected that this review can provide a profound guidance for more specialized design of defect-engineered catalysts with high activity and stability for nitrogen photochemical fixation.