Hybrid poly‐ion complexes were synthesized through the complexation of a double hydrophilic copolymer with Ce(III) ions. These colloids act as reservoirs for cerium ions, enabling the synthesis of cerium‐based Prussian blue nanoparticles with a cubic structure, a narrow size distribution around 100 nm, and good colloidal stability in water. Upon high‐temperature calcination, these nanoparticles are transformed into a cerium/iron‐based metal oxide catalyst (CeO2/Fe2O3). The resultant composite catalyst demonstrates superior performance in the photo‐Fenton oxidation of methylene blue pollutants, achieving a conversion efficiency that rivals other metal‐based oxides and cerium‐based catalysts.