The ideal liquid–solid heterogeneous electrocatalysis should have not only high catalytic activity but also free electron transport. However, preparing a single catalyst that simultaneously possesses both advantages has proven to be challenging. Herein, we prepared nano–micro composite catalysts (NMCCs) composed of highly dispersed Fe₃O₄ nanoparticles fixed on reduced graphene oxide (RGO) sheets (namely Fe₃O₄@RGO-NMCC) as the counter electrode (CE) in dye-sensitized solar cells (DSCs). Compared with the Fe₃O₄ or RGO CE, the Fe₃O₄@RGO-NMCC CE exhibited improved activity and reversibility for the catalytic reduction of triiodide ions (I₃ ⁻) to iodide ions (I⁻). Notably, DSCs using rigid and flexible Fe₃O₄@RGO-NMCC CEs achieved high PCEs up to 9% and 8% on fluorine-doped tin oxide (FTO)/glass substrates and flexible polymer substrates, respectively. These values are, to our knowledge, some of the highest reported efficiencies for DSCs based on a flexible Pt-free CE. We ascribed the superior catalytic performance of Fe₃O₄@RGO-NMCC to faster electron hopping between Fe²⁺ and Fe³⁺ and free electron transport by broad RGO sheets. Finally, Fe₃O₄@RGO-NMCC exhibited good stability in the practical application of DSCs because Fe₃O₄ nanoparticles were chemically bonded to the surface of RGO. Our work here will be of great interest for fundamental research and practical applications of Fe₃O₄ in lithium batteries, splitting water and magnetic fields.