The　effects　of　promoters　on　Ru　nanoparticles　(＞=　2　nm)　catalysts　for　NH3　synthesis　have　been　extensively　elaborated,　but　their　roles　on　ultrasmall　Ru　nanoclusters　(NCs,　1-2　nm)　remain　largely　unknown　and　need　to　be　further　uncovered.　Herein,　a　series　of　K-promoted　MgO　supported　Ru　NCs　were　synthesized　and　investigated　for　NH3　synthesis.　The　addition　of　5wt.%K　onto　Ru/MgO　NCs　leads　to　a　significantly　high　NH3　synthesis　rate　of　21.7　mmol(NH3)　g(cat)(-1)h(-1)　at　400　degrees　C　and　0.2　MPa,　close　to　the　thermodynamic　equilibrium.　Out　studies　reveal　that　anchoring　K　onto　Ru　NCs　can　increase　the　electron　density　and　cause　an　upshift　of　the　d-band　center　of　Ru　entities.　Moreover,　the　addition　of　K　regulates　the　hydrogen　affinity　and　accelerates　the　migration　of　hydrogen　from　the　Ru　NCs　surface　to　MgO　support,　which　is　crucial　in　avoiding　the　hydrogen　poisoning　effect　on　Ru　NCs.　With　the　synergistic　effect　of　the　Ru　NCs　sites　bridged　by　H-spillover,　makes　the　K-mediated　Ru/MgO　NCs　catalysts　efficient　for　NH3　synthesis　at　mild　conditions.