Ni-based oxide materials are promising candidates for catalyzing the oxygen evolution reaction. The detailed mechanism of water splitting in these systems has been of interest with a goal of understanding the intermediate species vital for catalytic activity. A potential intermediate species prior to release of oxygen is a bridging NiIII2(μ-1,2-peroxo) complex. However, Ni2(μ-1,2-peroxo) complexes are rare in general and are unknown with oxidation states higher than NiII. Herein, we report the isolation of such an unusual high-valent species in a NiIII2(μ-1,2-peroxo) complex, which has been characterized using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV-vis spectroscopies. In addition, treatment with excess tetrabutylammonium chloride results in regeneration of the precursor Ni–Cl species, implicating the reversible release of oxygen or a reactive oxygen species. Taken together, this suggests that NiIII2(μ-1,2-peroxo) species are accessible and may be viable intermediates during the oxygen evolution reaction.