Due to the large radius of K-ion, K-ion batteries (KIBs) involving K-ion insertion and extraction processes generally exhibit an insufficient cycle life and poor rate performance. In this work, sulphur-doped reduced graphene oxide (S-RGO) sponges, which are used as free-standing anodes for KIBs, are obtained via simple freeze-drying of graphene oxide solution and subsequent thermal treatment in sulphur steam. After sulphur doping, our S-RGO sponges deliver preeminent electrochemical performance. Such free-standing electrodes exhibit high K-ion storage capacity of 361 mAh g−1 at 50 mA g−1 over 50 cycles. Furthermore, even at a high current density of 1 A g−1, they also display a highly stabile K-ion storage with a capacity of 229 mAh g−1 over 500 cycles, which outperforms all previously reported carbon-based materials for KIBs. Such preeminent performance is attributed to the S-RGO sponges’ unique conductive structure and the sulphur doping, which effectively enhance the insertion of large K ions. We also utilize ex-situ XPS to illuminate the reaction process of our sample during potassiation/depotassiation and highlight the role of sulphur doping in improving K-ion storage performance.