A highly efficient photocatalytic system for hydrogen evolution with dihydronicotinamide coenzyme (NADH) as a sacrificial agent in an aqueous solution has been constructed by using water-soluble platinum clusters functionalized with methyl viologen-alkanethiol (MVA(2+)) and a simple electron-donor dyad, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), which is capable of fast photoinduced electron transfer but extremely slow back electron transfer. The mean diameter of the platinum core was determined as R-CORE = 1.9 nm with a standard deviation sigma = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogen-evolution rate of the photocatalytic system with MVA(2+)-modified platinum clusters (MVA(2+)-PtC) is 10 times faster than the photocatalytic system with the mixture of the same amount of MVA(2+) and platinum clusters as that of MVA(2+)-PtC under otherwise the same experimental conditions. The radical cation of NADH has been successfully detected by laser flash photolysis experiments. The decay of the absorbance due to NAD(center dot), produced by the deprotonation from NADH(center dot+), coincides with the appearance of the absorption band due to Acr(center dot)-Mes. This indicates electron transfer from NAD(center dot) to Acr(+)-Mes to give Acr(center dot)-Mes, which undergoes the electron-transfer reduction of MVA(2+)-PtC, leading to the efficient hydrogen evolution.