Mackinawite (FeS) nanoparticles have been extensively tested for reducing contaminants under anoxic conditions, while the oxidizing impact induced by FeS under oxic conditions has been largely underestimated. In light of previous findings that hydroxyl radicals (·OH) can be produced from oxygenation of sediment Fe(II), herein we revealed that ·OH can be produced efficiently from FeS oxygenation at circumneutral conditions, yielding 84.7 μmol ·OH per g FeS. Much more ·OH was produced from the oxygenation of FeS compared with siderite, pyrite, and zerovalent iron nanoparticles under the same conditions. The oxidation of FeS was a surface-mediated process, in which O2 was transformed by the structural Fe(II) on FeS surface to ·OH with the generation of H2O2 intermediate. A small proportion of Fe(II) was regenerated from the reduction of Fe(III) by FeS and S(-II), but this proportion did not significantly contribute to ·OH production. We further validated that the ·OH produced from FeS oxygenation considerably contributed to the oxidation of arsenic. As the change of redox conditions from anoxic to oxic is common in both natural and artificial processes, our findings suggest that the oxidizing impact induced by FeS at oxic conditions should be concerned due to ·OH production.