Since the first exfoliation and identification of graphene in 2004, research on layered ultrathin two-dimensional (2D) nanomaterials has achieved remarkable progress. Realizing the special importance of 2D geometry, one may naturally anticipate that the controlled synthesis of non-layered nanomaterials in 2D geometry could yield some unique properties that otherwise cannot be achieved in these non-layered systems. Herein, we report a systematic study involving theoretical and experimental approaches to evaluate the intercalation pseudocapacitive energy storage capability in 2D atomic sheets of non-layered molybdenum dioxide (MoO2). The uniform ultrathin 2D MoO2 are synthesized using a monomer-assisted, growth-confined solution approach. When used as electrodes for symmetrical microsupercapacitors, with PVA/LiCl gel electrolyte, these quasi-all-solid-state devices exhibit high areal capacitance (63.1 mF cm-2 at 0.1 mA cm-2), good rate performance (81% retention from 0.1 to 2 mA cm-2), and superior cycle stability (86% retention after 10,000 cycles), superior to its bulk counterparts. Furthermore, we show that capacitor-like charge storage in ultrathin 2D-MoO2 occurs to a much greater extent compared to its un-exfoliated analogue, implying faster kinetics for Li storage. In addition, in comparison with other state-of-the-art systems, our devices offer the best performance in terms of volumetric energy and power density in the parallel-plate configuration (17.2 mWh cm-3 at 0.35 W cm-3). More importantly, the present method is quite general and can be used to topotactically synthesize other low-valence-state metal oxides/sulfides (or even metals) with unique architecture. We believe that our work identifies a new pathway to make 2D nanostructures from non-layered compounds, which results in an extremely enhanced energy storage capability.