Perfluorooctanesulfonic acid (PFOS) has been used in various industries due to its excellent surfactant property and oxidative resistance. However, toxicology studies have shown that PFOS can cause adverse effects on ecological and human health. This study investigated the use of a MnO₂/H₂O₂ process to generate reactive radicals for PFOS removal. Our results indicated that among the three MnO₂ polymorphs (α-, β-, and γ-MnO₂), γ-MnO₂ was more effective in catalyzing H₂O₂ to remove PFOS. 0.25 μM PFOS could be completely degraded in 15 min under the optimal conditions ([H₂O₂] = 1 M, [γ-MnO₂] = 20 mg L⁻¹, initial pH = 7). Electron paramagnetic resonance (EPR) verified the formation of hydroxyl radicals (OH˙) and superoxide radicals (O₂˙⁻) and X-ray photoelectron spectroscopy (XPS) revealed the evolutions of the Mn oxidation state and O coordination environment on the MnO₂ surfaces in this process. Radical quenching experiments and intermediate identification indicated that the degradation of PFOS was initiated by the cleavage of the C–S bond, followed by that of the C–C bond and the C–F bond, leading to the formation of short-chain and defluorinated intermediates. Long-chain by-products were also identified, likely due to the recombination of organic radicals.