Manganese oxides (MnO2) are a class of promising electrode materials for high-performance supercapacitors. Among various possible crystal phases of MnO2, the two-dimensional layered birnessite (δ-MnO2) is considered to facilitate cation intercalation/deintercalation with little structural rearrangement during the charging and discharging process. Here, we report a facile strategy for synthesis of δ-MnO2 with significantly enhanced electrochemical capacitive energy storage performance via phase transformation of orthorhombic MnO2. The resultant δ-MnO2 delivers a specific capacitance of 251.4 F g−1 at a current density of 1 A g−1, which is almost three times higher than that of the original orthorhombic phase. The solid-state flexible asymmetric supercapacitor based on birnessite MnO2 as the positive electrode possesses an operating potential window as high as 2.0 V as well as high energy density and power density. This work demonstrates the post crystal engineering of MnO2 is an effective way to optimize their electrochemical properties.