Here, we utilize the strategy of anchoring single-atom cobalt sites onto the carbon nitride surface (Co-CN) to develop a fully heterogeneous photocatalytic system for the selective semihydrogenation of acetylene to ethylene that uses water as a proton source. The Co-CN has the CN support that generates electron-hole pairs under visible-light irradiation and hosts the Co SACs, thus effectively functioning as an all-in-one photocatalyst, in line with first-principles modelling. We demonstrate that the Co-CN photocatalyst converts acetylene to ethylene in water with selectivity >99.9 % selectivity for ethylene and retains its activity for 960 working hours. The heterogeneous nature of CN allows easy isolation of Co-CN, which can be recycled and reused five times without evidence of over-hydrogenation. Semi-hydrogenation of acetylene to ethylene: a) the state-of-the-art process that uses noble-metal catalysts and H2 at high temperature; b) recent photocatalytic hydrogenation routes that use cobalt catalysts in tandem with a photosensitizer and water or hydrogenated organics as proton donor at room temperature; c) this work that uses an all-in-one photocatalyst of single atom cobalt supported on carbon nitride and water as proton donor at room temperature for the >99.9 % selective production of ethylene.