Porously assembled 2D nanosheets of alkali metal manganese oxides were synthesized via the flocculation of exfoliated MnO2 nanosheets with alkali metal cations. According to X-ray diffraction and electron microscopic analyses, the MnO2 nanosheets were porously restacked with alkali metal cations, resulting in the mesoporous assembly of lamellar crystallites with surface expansion (∼ 50-70 m2 g-1). Mn K-edge X-ray absorption spectroscopy clearly demonstrated that manganese ions in the reassembled materials were stabilized in octahedral symmetry with the mixed oxidation state of Mn3+/Mn4+. The present reassembled manganates showed large capacitances of ∼140-160 F g-1 and excellent cyclability of ∼93-99% up to the 1000th cycle. The electrochemical cycling did not induce notable frustration in the crystal structure of manganate nanosheets, underscoring the high structural stability of the reassembled manganates. This study provided strong evidence for the effectiveness of the exfoliation- reassembling method in enhancing the capacitance performance of layered metal oxides.