Abstract The development of manganese-based water oxidation electrocatalysts is desirable for the production of solar fuels, as manganese is earth-abundant, inexpensive, non-toxic, and has been employed by the Photosystem?II in nature for a billion years. Herein, we directly constructed a 3?D nanoarchitectured turbostratic δ-MnOx on carbon nanotube-modified nickel foam (MnOx/CNT/NF) by electrodeposition and a subsequent annealing process. The MnOx/CNT/NF electrode gives a benchmark catalytic current density (10?mA?cm?2) at an overpotential (?) of 270?mV under alkaline conditions. A steady current density of 19?mA?cm?2 is obtained during electrolysis at 1.53?V for 1.0?h. To the best of our knowledge, this work represents the most efficient manganese-oxide-based water oxidation electrode and demonstrates that manganese oxides, as a structural and functional model of oxygen-evolving complex (OEC) in Photosystem?II, can also become comparable to those of most Ni- and Co-based catalysts.