Advanced oxidation processes (AOPs) are efficient methods for water purification. However, there are few studies on direct oxidation with inactive peroxymonosulfate (PMS) to remove pollutants. In this study, methylene blue (MB) was oxidised efficiently by PMS. MB was decomposed through a non-radical pathway, as verified by the scavengers and electron paramagnetic resonance experiments. Although singlet oxygen was formed, the possibility of it degrading MB was ruled out via kinetics calculations. Therefore, the obvious conclusion was that PMS reacted directly with MB. Additionally, the operating parameters, including PMS dosage, MB dosage, temperature, and initial pH, were investigated. MB degradation followed pseudo-first-order kinetics, with rate constants ranging from 0.0082 to 0.3912 min−1. The second-order rate constant (kPMS,MB) for the reaction of MB with PMS was 0.08 M−1s−1 at pH 3–6, but kPMS,MB increased dramatically to 4.68 M−1s−1 with increasing solution pH. Chloride in the PMS/MB system reacted with PMS to form HClO, which was catalysed by PMS to rapidly decompose MB. Interestingly, MB is effectively mineralised by PMS in the presence of chlorine. In addition, humic acid, a common scavenger, did not inhibit MB degradation. In an alkaline environment, the removal rate of TOC reached 16%, but PMS caused almost no mineralisation of MB in an acidic environment. This study provides new insights into pollutant removal based on AOPs and an additional strategy for water purification.