The electrocatalytic nitrogen reduction reaction (NRR) has been one of the most intriguing catalytic reactions in recent years, providing an energy-saving and environmentally friendly alternative to the conventional Haber-Bosch process for ammonia production. However, the activity and selectivity issues originating from the activation barrier of the NRR intermediates and the competing hydrogen evolution reaction result in the unsatisfactory NH₃ yield rate and Faradaic efficiency of current NRR catalysts. Atomic site catalysts (ASCs), an emerging group of heterogeneous catalysts with a high atomic utilization rate, selectivity, and stability, may provide a solution. This article undertakes an exploration and systematic review of a highly significant research area:the principles of designing ASCs for the NRR. Both the theoretical and experimental progress and state-of-the-art techniques in the rational design of ASCs for the NRR are summarized, and the topic is extended to double-atom catalysts and boron-based metal-free ASCs. This review provides guidelines for the rational design of ASCs for the optimum activity and selectivity for the electrocatalytic NRR.