The increase in the amount of wastewater containing organic pollutants in various industrial processes creates serious problems for the environment. Sulfate radical-based advanced oxidation process (AOP) is an effective route to remove pollutants from wastewater. However, designing a new nano-based catalyst to generate sulfate radicals is an important factor for the AOP. For this vision, porous trimanganese tetraoxide-polydicyclopentadiene (Mn3O4/pDCPD) nanocomposite, having an open-cell structure, was successfully designed via high internal phase emulsion (HIPE) and ring-opening metathesis polymerization (ROMP) approaches. The effect of Mn3O4 nanoparticle concentration on the structure was investigated, and the resulting Mn3O4/p(DCPD)HIPE nanocomposites were fully characterized by FT-IR, XRD, FE-SEM, TEM, solid-state 13C CPMAS NMR, DSC, and TGA analysis. The selected nanocomposite containing 5 wt% of Mn3O4 was used as a model catalyst to mediate the heterogeneous oxidation of phenol in the presence of oxone. It is concluded that Mn3O4/p(DCPD)HIPE nanocomposite is a highly active catalyst to generate sulfate radicals for phenol degradation. Complete removal of 25 mg/L phenol was achieved in 30 min under the conditions of [catalyst] = 0.8 g/L, [oxone] = 2 g/L, and T = 25 °C. The phenol degradation followed the pseudo-first-order kinetic model, and the highest kinetic constant of 0.0611 min−1 was achieved. No significant loss in the activity of the catalyst was determined after four consecutive cycles.