The full reaction mechanism of nitrite hydratase has remained elusive, despite extensive theoretical and experimental studies. A novel reaction mechanism for nitrile hydratase is proposed here, with remarkable features and very feasible barriers. Our results, obtained on the basis of large quantum-mechanical active site models, identify Cys-SO- as the nucleophile, performing a direct nucleophilic attack on the metal-coordinated nitrile. This implies the formation of an intriguing cyclic intermediate, which subsequently is cleaved through attack of the axial cysteine on the sulfenate, thereby forming a disulfide bond. In this mechanism, nitrile hydration occurs without directly involving a water molecule. Subsequent water-mediated disulfide cleavage regenerates the active site. This is the first example of a disulfide switch directly implicated in an enzymatic reaction mechanism.