Electrochemical water splitting is one of the ways to produce environmentally‐friendly hydrogen energy. Transition‐metal (TM)‐based catalysts have been attracting attention due to their low cost and abundance, but their insufficient activity still remains a challenge. Here, 4 nm Mn₃O₄ nanoparticles (NPs) are successfully synthesized and their electrochemical behavior is investigated. Using electrokinetic analyses, an identical water oxidizing mechanism is demonstrated between the 4 and 8 nm Mn₃O₄ NPs. In addition, it is confirmed that the overall increase in the active surface area is strongly correlated with the superb catalytic activity of the 4 nm Mn₃O₄ NPs. To further enhance the oxygen evolution reaction (OER) performance, Ni foam substrate is introduced to maximize the entire number of the NPs participating in OER. The 4 nm Mn₃O₄/Ni foam electrode exhibits outstanding electrocatalytic activity for OER with overpotential of 395 mV at a current density of 10 mA cm⁻² under neutral conditions (0.5 m PBS, pH 7).