Hierarchical layered K0.27MnO2 microflowers are firstly synthesized via a facile and efficient route based on a topochemical reaction process. As cathode materials for aqueous sodium-ion battery, such microflowers exhibit high reversible capacity, long cyclic life and excellent rate capability. After 100 cycles, a reversible capacity of 68.5mAhg–1 at a current density of 0.2Ag–1 is attained in a full cell with K0.27MnO2 as cathode and NaTi2(PO4)3 as anode. We propose a sodium storage mechanism in K0.27MnO2 by analyzing the evolution of structure and interface during charge/discharge process.