The photocatalytic function of three-dimensional porphyrin monolayer-protected gold clusters (MFCs) with different chain lengths has been examined in photocatalytic reduction of hexyl viologen (HV2+) by 1-benzyl-1,4-dihydronicotinamide (BNAH) in comparison with that of the reference porphyrin compound without metal clusters. Both porphyrin monolayer-protected gold clusters and the reference porphyrin compound act as efficient photocatalysts for the uphill reduction of HV2+ by BNAH to produce 1-benzylnicotinamidinium ion (BNA+) and hexyl viologen radical cation (HV•+) in benzonitrile. In the case of porphyrin monolayer-protected gold clusters the quantum yield reaches a maximum value with an extremely low concentration of HV2+, which is larger than the corresponding value of the reference porphyrin compound. The dependence of quantum yields on concentrations of BNAH and HV2+ as well as the time-resolved single-photon-counting fluorescence and transient absorption spectroscopic results indicates that the photoinduced electron transfer from the triplet excited state of the reference porphyrin to HV2+ initiates the photocatalytic reduction of HV2+ by BNAH, but that the photoinduced electron transfer from the singlet excited state of porphyrin monolayer-protected gold clusters to HV2+, which forms complexes with MFCs, is responsible for the photocatalytic reaction. The intersystem crossing from the porphyrin singlet excited state to the triplet is much suppressed by the quenching of the porphyrin excited singlet state via energy transfer to the gold surface of the three-dimensional MFCs. However, the three-dimensional architectures of porphyrin MFCs with large surface areas allow HV2+ to interact with MFCs, resulting in fast electron transfer from the singlet excited state of porphyrin to HV2+ on MFCs. This is the reason the quantum yield of the photocatalytic reduction of HV 2+ by BNAH reaches a maximum value at an extremely small concentration of HV2+ when the surface of MFCs is covered by HV 2+. The light-harvesting efficiency of MFCs is much improved as compared with the reference compound.