Aqueous rechargeable microbatteries are promising on-chip micropower sources for a wide variety of miniaturized electronics. However, their development is plagued by state-of-the-art electrode materials due to low capacity and poor rate capability. Here we show that layered potassium vanadium oxides, KxV2O5·nH2O, have an amorphous/crystalline dual-phase nanostructure to show genuine potential as high-performance anode materials of aqueous rechargeable potassium-ion microbatteries. The dual-phase nanostructured KxV2O5·nH2O keeps large interlayer spacing while removing secondary-bound interlayer water to create sufficient channels and accommodation sites for hydrated potassium cations. This unique nanostructure facilitates accessibility/transport of guest hydrated potassium cations to significantly improve practical capacity and rate performance of the constituent KxV2O5·nH2O. The potassium-ion microbatteries with KxV2O5·nH2O anode and KxMnO2·nH2O cathode constructed on interdigital-patterned nanoporous metal current microcollectors exhibit ultrahigh energy density of 103 mWh cm-3 at electrical power comparable to carbon-based microsupercapacitors.