The peroxone reaction between O-3 and H2O2 has been deemed a promising technology to resolve the increasingly serious water pollution problem by virtue of the generation of superactive hydroxyl radicals ((OH)-O-center dot), but it suffers greatly from an extremely limited reaction rate constant under acidic conditions (ca. less than 0.1 M-1 s(-1) at pH 3). This article describes a heterogeneous catalyst composed of single Mn atoms anchored on graphitic carbon nitride, which effectively overcomes such a drawback by altering the reaction pathway and thus dramatically promotes (OH)-O-center dot generation in acid solution. Combined experimental and theoretical studies demonstrate Mn-N-4 as the catalytically active sites. A distinctive catalytic pathway involving HO2 center dot formation by the activation of H2O2 is found, which gets rid of the restriction of HO2- as the essential initiator in the conventional peroxone reaction. This work offers a new pathway of using a low-cost and easily accessible single-atom catalyst (SAC) and could inspire more catalytic oxidation strategies.