Inspired by the natural oxygen evolution reaction of Photosystem II, the earth-abundant and inexpensive manganese oxides (MnOx) have been recognized for their great potential as highly efficient and robust materials for water oxidation reaction (WORs). To date, most of the heterogeneous, synthesized MnOx catalysts still exhibit lower activities for WORs, in comparison to RuO2 and IrO2. Herein, we report a single-step and scalable synthesis method for mesoporous MnOx materials that is developed through a soft-templated method. This method allowed precise control of Mn3+-rich Mn2O3 structure as well as pore sizes and crystallinity of these mesoporous MnOx. These catalysts were investigated for both photochemical and electrochemical water oxidation, and they presented a superior activity for water oxidation. The highest turnover frequency of 1.05 × 10–3 s–1 was obtained, which is comparable with those for precious metal oxide based catalysts (RuO2 and IrO2). Our results illustrate a guideline to the design and synthesis of inexpensive and highly active heterogeneous catalysts for water oxidation.