Water splitting into H-2 and O-2 under visible light was achieved using simple organic dyes such as coumarin and carbazole as photosensitizers on an n-type semiconductor for H-2 evolution, a tungsten(VI) oxide (WO3) photocatalyst for O-2 evolution, and a triiodide/iodide (I-3(-)/I-) redox couple as a shuttle electron mediator between them. The results on electrochemical measurements revealed that the oxidized states of the dye molecules having an oligothiophene moiety (two or more thiophene rings) in their structures are relatively stable even in water and possess sufficiently long lifetimes to exhibit reversible oxidation-reduction cycles, while the carbazole system required more thiophene rings than the coumarin one to be substantially stabilized. The long lifetimes of the oxidized states enabled these dye molecules to be regenerated to the original states by accepting an electron from the I- electron donor even in an aqueous solution, achieving sustained H-2 and I-3(-) production from an aqueous KI solution under visible light irradiation when they were combined with an appropriate n-type semiconductor, ion-exchangeable layered niobate H4Nb6O17. The use of H4Nb6O17 loaded with Pt cocatalyst inside the interlayer allowed the water reduction to proceed preferentially with a steady rate even in the presence of a considerable amount of I-3(-) in the solution, due to the inhibited access of I-3(-) to the reduction site, Pt particles inside, by the electrostatic repulsion between the I-3(-) anions and the negatively charged (Nb6O17)(4-) layers. It was also revealed that the WO3 particles coloaded with Pt and IrO2 catalysts exhibited higher rates of O-2 evolution than the WO3 particles loaded only with Pt in aqueous solutions containing a considerable amount of I-, which competitively consumes the holes and lowers the rate of O-2 evolution on WO3 photocatalysts. The enhanced O-2 evolution is certainly due to the improved selectivity of holes toward water oxidation on IrO2 cocatalyst, instead of undesirable oxidation of I-. Simultaneous evolution of H-2 and O-2 under visible light was then achieved by combining the Pt/H4Nb6O17 semiconductor sensitized with the dye molecules having an oligothiophene moiety, which can stably generate H-2 and I-3(-) from an aqueous KI solution, with the IrO2-Pt-loaded WO3 photocatalyst that can reduce the I-3(-) back to I- and oxidize water to O-2.