Ammonia (NH₃) synthesis is an important industrial chemical process. Recently, electrochemically converting the earth-abundant dinitrogen (N₂) in the aqueous phase to NH₃ at ambient conditions has been proposed as an alternative to the well-established Haber–Bosch process. Catalysts for the electrochemical N₂ reduction to NH₃ play crucial roles in realizing this NH₃ synthesis route. Electrochemical N₂ reduction has been studied for decades, and many studies have emerged in the past few years. Herein, we provide a comprehensive review to summarize various catalysts used for achieving electrochemical N₂ reduction to NH₃, including homogeneous, heterogeneous and biological catalysts, as well as relevant computational studies to understand their reaction mechanisms. We compare the advantages and shortcomings of these catalytic systems. Future research directions for realizing catalysts with low overpotentials, high energy efficiency, good scalability, and stability modularity are also proposed. This review provides an overview of this fast-growing research field and encourages more studies toward the rational design of catalysts for electrochemical N₂ reduction to NH₃ under ambient conditions.