Although hydrogen plays a crucial role in ammonia synthesis, very little is known about its poisoning of Ru catalysts. In this study, density functional theory calculations of H₂ and N₂ dissociations, and H atom binding on Ru₁₅₃ were performed to provide a fundamental understanding of hydrogen poisoning. Because of the kinetic dominance of the H₂ dissociation over N₂ (vertically or horizontally adsorbed) splitting, the dissociated H atoms block the active sites required for horizontal (less energetically demanding dissociation) N₂ adsorption to occur either from the gas phase or after its geometrical transformation from being adsorbed vertically. Additionally, the dissociated H atoms withdraw electrons from the surface, which reduces the ability of the neighboring Ru atoms to donate electrons for N₂ activation, hindering its dissociation and suppressing ammonia synthesis.