We report the first example of a mononuclear nonheme manganese(III)-hydroperoxo complex derived from protonation of an isolated manganese(III)peroxo complex bearing an N-tetramethylated cyclam (TMC) ligand, [Mn-III(TMC)(OOH)(2+). The Mn-III-mhydroperoxo intermediate is characterized with various spectroscopic methods as well as with density functional theory (DFT) calculations, showing the binding of a hydroperoxide ligand in an end-on fashion. The Mn-III-hydroperoxo species is a competent oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of sulfides. The electrophilic character of the Mn-III-hydroperoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles.