Microplastic (MP) pollution in water has increased in the last decade, driving the development of technologies for its treatment. The degradation of MP is an effective alternative, since it eliminates them and prevents their accumulation in the environment. In this study, the degradation of polyethylene microplastics (PE-MP) by wet oxidation (WO) in the presence of peroxymonosulfate (PMS, 0.75 mM and 300 mM) and sulfuric acid (H2SO4, 0.0 M, 4.0 M, 7.5 M, and 11.4 M) at different temperatures was evaluated. A Fenton-like reaction was also analyzed, adding Co2+ (0.0 mM and 0.8 mM) to catalyze the decomposition of PMS at 120 °C. Degradation was monitored using chemical oxygen demand (COD) related to the release of MP-derived components, with a significant increase observed after 2 h with Co2+, followed by a reduction at 6 h, suggesting partial mineralization or formation of volatile compounds. The results indicate that PE-MPs are complex, refractory particles whose degradation requires the combination of autogenous pressure, temperature, acidic conditions, and free radicals (i.e., sulfate radicals produced by the breakdown of PMS). This methodology is presented as a promising alternative for the degradation of MP in the aqueous phase.