Electrochemical　nitrogen　reduction　reaction　(eNRR)　is　an　alternative　promising　manner　for　sustainable　N2　fixation　with　low-emission.　The　major　challenge　for　developing　an　efficient　electrocatalyst　is　the　cleaving　of　the　stable　N[tbnd]N　triple　bonds.　Herein,　we　design　a　new　MoS2　with　in-plane　defect　cluster　through　a　bottom-up　approach　for　the　first　time,　where　the　defect　cluster　is　composed　of　three　adjacent　S　vacancies.　The　well-defined　in-plane　defect　clusters　could　contribute　to　the　strong　chemical　adsorption　and　activation　towards　inert　nitrogen,　achieving　an　excellent　eNRR　performance　with　an　ammonia　yield　rate　of　43.4　±　3　μg　h−1　mgcat.−1　and　a　Faradaic　efficiency　of　16.8　±　2%　at　−0.3　V　(vs.　RHE).　The　performance　is　much　higher　than　that　of　MoS2　with　the　edge　defect.　Isotopic　labeling　confirms　that　N　atoms　of　produced　NH4+　originate　from　N2.　Furthermore,　the　in-plane　defect　clusters　realized　the　alternate　hydrogenation　of　nitrogen　in　a　side-on　way　to　synthesize　ammonia.　This　work　provides　a　prospecting　strategy　for　fine-tuning　in-plane　defects　in　a　catalyst,　and　also　promotes　the　progress　of　eNRR.　©　2021　Science　Press