The indirect synthesis of NH3 from N-2 and H2O through plasma processing is proposed and demonstrated. NH3 is a promising hydrogen storage material because of its high hydrogen storage density. Mg3N2 is a key material for indirect NH3 synthesis, because the reaction of Mg3N2 with water easily generates NH3 at room temperature. In this paper, therefore, we focus on generating Mg3N2 by nitridation of MgO with nonthermal atmospheric-pressure dielectric barrier discharge (DBD) plasma in a N-2 atmosphere. By intermittent DBD nitridation treatment while the reaction device was cooled in a water bath, a maximum Mg3N2 generation efficiency of 93 mg/kWh was estimated. Because NH3 is generated through a simple chemical reaction, our scheme does not cause NH3 decomposition by plasma, which is one of the greatest concerns associated with plasma synthesis. Contrary to the conventional NH3 generation process, which emits CO2 and requires high temperature and pressure, our scheme enables NH3 synthesis from N-2 and H2O without CO2 emissions. This allows for an onsite small-scale NH3 synthesis system to be realized under mild conditions, which is necessary for a future low-carbon society.