We report the first synthesis of highly homogenous Ce-doped YAG/ZnO core/shell nanoparticles (YAG:Ce/ZnO CSN) based on the hydrolysis/condensation of Zn(OAc)2 on the surface of YAG:Ce nanoparticles (NPs). Results show that YAG:Ce NPs of about 100 nm diameter are homogenously surrounded by a multilayer of highly crystallized ZnO nanocrystals (ZnO NCs) of 10–15 nm diameter with a core/shell structure. The as-prepared nanostructures have been used in the photocatalytic degradation of sulfathiazole (STZ), which is a molecule widely used as antibiotic, under UV-vis and visible light. The effect of YAG:Ce/ZnO weight ratio and YAG:Ce particle size on the photocatalytic efficiency of YAG:Ce/ZnO core/shell structures has been studied. The YAG:Ce/ZnO weight ratio of 1/1 was found to yield the optimal photocatalytic activity. Results also showed that YAG:Ce/ZnO CSN with 100 nm core size exhibited much higher photocatalytic activity compared to YAG:Ce/ZnO CSN with micro-sized YAG;Ce core. The recyclability of YAG:Ce/ZnO CSN photocatalyst was also demonstrated over at least 10 photocatalytic degradation cycles.