Photocatalytic carboxylation of acetylacetone with carbon dioxide has been performed by using ZnS-based photocatalysts. The formation of two isomeric carboxylic acids, as proven by IR and 13C NMR spectroscopy, was observed. The reaction yield was enhanced after deposition of ruthenium nanoparticles on ZnS. The reaction encompasses ruthenium-mediated one-electron reduction of CO2 to CO2.− with electrons from the conduction band of ZnS and one-hole oxidation of acetylacetone to the relevant radical. Coupling of photogenerated radicals leads to the formation of carboxylic acids. Generation of CO2.− has been confirmed by spin-trapping EPR measurements. The process described herein may find applications for the solar-light-driven green synthesis of Cn+1 carboxylic compounds from Cn substrates by utilising carbon dioxide.