© 2020 Wiley-VCH GmbH Inorganic/organic dielectric nanocomposites have been extensively explored for energy storage applications for their ease of processing, flexibility, and low cost. However, achieving simultaneous high energy density and high efficiency under practically workable electric fields has been a long-standing challenge. Guided by first-principles calculations of interface properties and phase-field simulations of the dynamic dielectric breakdown process, superhierarchical nanocomposites of ferroelectric perovskites, layered aluminosilicate nanosheets, and an organic polymer matrix are designed and simultaneous high energy density of 20 J cm−3 and high efficiency of 84% at a low electric field of 510 MV m−1 are achieved. This is the highest energy density of all the state-of-the-art dielectric polymer nanocomposites with energy efficiency > 80% at a low electric field of <600 MV m−1. Strong atomic hybridization, large ionic displacement, the enhanced breakdown strength through forming charge-blocking layers, and the superhierarchical microstructure with gradient interfaces are responsible for the high performances. This superhierarchical structuring modulation strategy is generally applicable to composites for different functionalities and applications.