In photoelectrochemical (PEC) water splitting, exploring highly active cocatalysts, especially Mn-based catalysts, is becoming more important in enabling a comparison of a number of Co-, Fe-, and Ni-based traditional cocatalysts in solar water oxidation. Herein, we for the first time synthesized an efficient Mn-based (oxy)hydroxide catalyst through MnSe as an intermetallic phase. As iron was further incorporated into the MnSe, the MnFeSe electrode showed an outstanding oxygen evolution reaction (OER) activity (an overpotential of 247 mV at 10 mA cm⁻²) and reaction kinetics (a Tafel slope of 35 mV dec⁻¹). The MnFeSe was electrochemically converted to Se-incorporated MnFe (oxy)hydroxides under oxidative potential during the OER. Remarkably, the novel Mn-based multi-metal catalyst was applied to BiVO₄ for solar-driven water splitting. The resulting photoelectrode exhibited a current density of 4.85 mA cm⁻² at 1.23 V_RHE in 1 M KBi under AM 1.5G illumination. More importantly, a 20 h durability was achieved owing to avoiding the photocorrosion of BiVO₄, which is so far the best PEC performance using a highly active Mn-based cocatalyst compared to the common MnO_x. Further DFT calculations on models of MnSe, MnFeSe, MnOOH, Se,Fe–MnOOH, MnO₂, and Se,Fe–MnO₂ explained well the OER/PEC experimental results. This work developed a new strategy for designing and exploring advanced cocatalysts for a practical solar-driven water splitting system.