Potassium-ion batteries attract tremendous attention for large scale application due to the abundance of K resources. Herein, a highly disordered hard carbon derived from a skimmed cotton is investigated for this purpose. The study shows that a simple soaking treatment in hydrochloric acid for the skimmed cotton before its high-temperature carbonization can critically impact the structure and the electrochemical properties of the obtained hard carbon significantly. This hard carbon exhibits a high initial coulombic efficiency (73%) and superior cycling stability and rate capability (253 mAh g−1 and 165 mAh g−1 at 40 mA g−1 and 4000 mA g−1, respectively) due to its unique porous architecture and large surface area. The full cell by coupling the hard carbon with a cathode material, i.e. potassium-rich iron hexacyanoferrate shows an outstanding electrochemical performance. These promising properties highlight the potentials of such a hard carbon in practical applications.