As a water oxidation catalyst, -Mn2O3 has higher activity than other manganese oxides. However, the robustness of the catalyst supported on conducting electrodes has only been tested for a short time (1h) and it shows a gradual decrease in activity. Furthermore, the turnover number (TON) and turnover frequency (TOF) have not been reported. Herein, we optimized the preparation of transparent nanocrystalline -Mn2O3 on a fluorine-doped tin oxide (FTO) electrode, and this resulting catalyst shows, in neutral aqueous 0.1m potassium phosphate buffer solution, a high electrocatalytic water oxidation activity with a TON of 230, a TOF of 5.3x10(-3)s(-1) based on the all-Mn content, and a TOF of 2.1s(-1) based on the amount of surface-active Mn with a Faradic efficiency of 96.7% at an overpotential of 470mV. The robustness of the -Mn2O3/FTO electrocatalyst was tested for a long time (80h) without a decrease in activity.