The electrocatalytic activity of transition-metal (TM)-based catalysts is correlated with the spin states of metal atoms. However, developing a way to manipulate spin remains a great challenge. Using first-principles calculations, we first report the crucial role of the spin of exposed Mo atoms around an S-vacancy in the electrocatalytic dinitrogen reduction reaction on defective MoS2 nanosheets and propose a novel strategy for regulating the electronic spin moments by tuning a single-atom promoter (SAP). Single TM atoms adsorbed on a defective MoS2 basal plane serve as SAPs via a noncontact interaction with an exposed Mo active site, inducing a significant spin polarization that promotes N2 adsorption and activation. Interestingly, by changing only the adsorption site of the TM atom, we are able to change the spin moments of the Mo atom, over a wide range of tunable values. The spin moments can be tuned to largely improve the catalytic activity of MoS2 toward the reduction of N2 to NH3.