Dielectric capacitors with ultrahigh power densities are fundamental energy storagecomponents in electrical and electronic systems. However, a long-standing challenge isimproving their energy densities. We report dielectrics with ultrahigh energy densitiesdesigned with polymorphic nanodomains. Guided by phase-field simulations, weconceived and synthesized lead-free BiFeO3-BaTiO3-SrTiO3solid-solution films to realizethe coexistence of rhombohedral and tetragonal nanodomains embedded in a cubicmatrix. We obtained minimized hysteresis while maintaining high polarization andachieved a high energy density of 112 joules per cubic centimeter with a high energyefficiency of ~80%. This approach should be generalizable for designing high-performance dielectrics and other functional materials that benefit from nanoscaledomain structure manipulation.