Water oxidation is the key step for water splitting to produce hydrogen. Synthesis of earth-abundant, efficient, and robust water oxidation catalysts is vital for photocatalytic and electrocatalytic water splitting. Herein, inspired from natural oxygen evolving site Mn4CaO5 cluster, we fabricated very small Mn3O4 nanoparticles onto the surface of sulfonated-graphite (denoted as "Mn3O4@G-Ph-SN") for photochemical and electrochemical water oxidation through a facile hydrothermal process. The supported Mn3O4 nanoparticles are homogeneously dispersed and the mean particle size is as small as 3.2 nm. The obtained hybrid exhibits efficient water oxidation activity with turnover frequency (TOF) of 0.1 s(-1), several orders of magnitude higher than commercial manganese-based oxide water oxidation catalysts. In addition, Mn3O4@G-Ph-SN shows electrochemical water oxidation performance with an overpotential of 473 mV in 1.0 M KOH solution and with good stability. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.