In the water‐splitting reaction, the oxidation of water to O2 is considered to be a kinetically demanding process, so that a co‐catalyst has been usually applied to promote water oxidation. This work demonstrates that, when loading mixed‐valence cobalt (II,III) oxide (Co3O4) dispersed in a Nafion membrane (Nf) on a nanoporous bismuth vanadate (BiVO4) photoanode (i. e., BiVO4/Nf[Co3O4]), stable and efficient water oxidation occurred. In particular, it is noteworthy that the BiVO4/Nf[Co3O4] photoanode exhibited stable performance even in an acidic medium (pH=2). In the present system consisting of a BiVO4/Nf[Co3O4] photoanode along with an organo‐photocathode, the stoichiometric formation of O2 and H2 successfully occurred by applying just 0.1 V between the photoelectrodes (cf. maximum efficiency, ca. 0.2 %), which was superior to the prototype comprising BiVO4 and Pt counter electrode. The Co3O4 loading can effectively suppress the collapse of the nanostructured BiVO4 upon photocorrosion, resulting in the stable and kinetically efficient consumption of holes in BiVO4.