A new mononuclear nickel(II) complex, [NiII(dpaq)Cl] (1), containing a tetradentate monoamido ligand, dpaq (dpaq=2‐[bis(pyridin‐2‐ylmethyl)amino]‐N‐(quinolin‐8‐yl)acetamide), has been synthesized and characterized by IR spectroscopy, elemental analysis, and UV/Vis spectroscopy. The structure of the nickel complex has been determined by X‐ray crystallography. This nonheme NiII complex 1 catalyzed the epoxidation reaction of a wide range of olefins with meta‐chloroperoxybenzoic acid (m‐CPBA) under mild conditions. Olefin epoxidation using this catalytic system has been proposed to involve a new reactive NiIV–oxo (4) species, based on the evidence from a PPAA (peroxyphenylacetic acid) probe, Hammett studies, H218O exchange experiments, and ESI mass spectroscopic analysis. Moreover, the nature of solvent significantly influenced partitioning between heterolytic and homolytic O−O bond cleavage of the Ni–acylperoxo intermediate (2). The O−O bond of 2 proceeded predominantly through heterolytic cleavage in a protic solvent, such as CH3OH. These results suggest that possibly a NiIV–oxo species is a common reactive intermediate in protic solvents. The two active oxidants, namely NiIV–oxo (3) and NiIII–oxo (4), which are responsible for stereospecific olefin epoxidation and radical‐type oxidations, respectively, operate in aprotic solvents.