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Nickel superoxide dismutase (NiSOD) is an enzyme that protects cells against O 2 · - . While the structure of its active site is known, the mechanism of the catalytic cycle is still not elucidated. Its active site displays a square planar Ni II center with two thiolates, the terminal amine and an amidate. We report here a bioinspired Ni II complex built on an ATCUN-like binding motif modulated with one cysteine, which demonstrates catalytic SOD activity in water ( k cat = 8.4(2) × 10 5 M -1 s -1 at pH = 8.1). Its reactivity with O 2 · - was also studied in acetonitrile allowing trapping two different short-lived species that were characterized by electron paramagnetic resonance or spectroelectrochemistry and a combination of density functional theory (DFT) and time-dependent DFT calculations. Based on these observations, we propose that O 2 · - interacts first with the complex outer sphere through a H-bond with the peptide scaffold in a [Ni II O 2 · - ] species. This first species could then evolve into a Ni III hydroperoxo inner sphere species through a reaction driven by protonation that is thermodynamically highly favored according to DFT calculations.