The synthesis of molecular host-guest complexes with enhanced performance, relative to those of their components, is a central theme in supramolecular chemistry. Here we explore a host-guest system consisting of an atomically precise gold nanocluster bound inside a zinc porphyrin nanoring. UV/Vis absorption and fluorescence titrations with different sized nanorings revealed strong binding between a pyridinethiol-coated Au₂₅ nanocluster and a nanoring consisting of six zinc porphyrin units, and complexation is confirmed by mass spectrometry. Formation of this assembly enhances the stability of the gold nanocluster. The host-guest complex also exhibits remarkable activity and selectivity for photochemical CO₂ to CO conversion and singlet oxygen generation.