Thiosemicarbazone-based complexes have been explored as a new class of redox-active catalysts H 2 production due to their flexibility for extensive optimization. To rationalize the process, we need to understand how these complexes function. In this work, we used DFT calculations to investigate the various mechanisms that could take place for three previously characterized Ni complexes. We found that two possible mechanisms are compatible with previously published experimental data, involving protonation of two adjacent N atoms close to the metal center. The first step likely involves a proton-coupled electron transfer process from a proton source to one of the distal N atoms in the ligand. From here, a second proton can be transferred either to the coordinating N atom situated in between the first protonated atom and the Ni atom, or to the second distal N atom. The former case then has the protons in close distance for H 2 production. However, the latter will require a third protonation event to occur, which would fall in one of the N atoms adjacent to the Ni center, resulting in a similar mechanism. Finally, we show that the H-H bond formation is the rate-limiting step, and suggest additional strategies that can be taken into account to further optimize these complexes. The reaction mechanism of nickel thiosemicarbazone complexes, a new class of redox-active catalysts for H 2 evolution, is investigated by the means of DFT methods.