Tungsten trioxide (WO3) powder was studied as a photocatalyst for water oxidation under visible light (λ > 420 nm). WO3 modified with nanoparticulate Pt species (more specifically, PtOx) as cocatalysts is capable of photocatalyzing water oxidation under visible light in the presence of iodate (IO3-) ions as an electron acceptor under near-neutral pH conditions (pH ≈ 5.9). When PtO x/WO3 was further modified with a very small amount (0.001 wt%) of a metal oxide (e.g., MnOx, CoOx, RuO2 or IrO2) as a secondary cocatalyst, the water oxidation activity was improved. Among the metal oxide cocatalysts examined, RuO2 was found to give the highest performance, with an apparent quantum yield of 14.4% at 420 nm. The results of photocatalytic reactions and photoelectrochemical analyses suggest that the main roles of the loaded PtOx and RuO2 on WO3 are to promote the reduction of IO3- and water oxidation, respectively.