Ammonia synthesis by using water as a hydrogen source is a challenging task. Laser-ablation-generated ScNH+ cations have been mass-selected using a quadrupole mass filter and reacted with H2O in a linear ion trap reactor under thermal collision conditions. Through mass spectrometry in conjunction with density functional theory calculations, we found that ammonia is released as the product in the reaction of ScNH+ with H2O, and this reaction is with high efficiency and selectivity, and the rate constant for the reaction is (1.14 +/- 0.23) x 10(-10) cm(3) molecule(-1) s(-1), corresponding to the reaction efficiency of 15%. Metal imido complexes (*MNH) are one of the important intermediates in the currently reported NH3 synthetic reactions. The gas-phase ScNH+ cation can be a simplified model of *MNH over catalysts of NH3 synthesis, and the facile proton transfer mechanism obtained in this model system may offer fundamental mechanistic insights into how to design catalysts for ammonia production by using water as the hydrogen source under ambient conditions.