AbstractMesoporous Co3O4 was prepared using a dual templating approach whereby mesopores inside SiO2 nanospheres, as well as the void spaces between the nanospheres, were used as templates. The effect of calcination temperature on the crystallinity, morphology, and textural parameters of the Co3O4 replica was investigated. The catalytic activity of Co3O4 for photochemical water oxidation in a [Ru(bpy)3]2+[S2O8]2− system was evaluated. The Co3O4 replica calcined at the lowest temperature (150 °C) exhibited the best performance as a result of the unique nanostructure and high surface area arising from the dual templating. The performance of Co3O4 with highest surface area was further examined in electrochemical water oxidation. Superior activity over high temperature counterpart and decent stability was observed. Furthermore, CoO with identical morphology was prepared from Co3O4 using an ethanol reduction method and a higher turnover‐frequency number for photochemical water oxidation was obtained.