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Abstract The unique nonlinear dielectric properties of antiferroelectric (AFE) oxides are promising for advancements in solid state supercapacitor, actuator, and memory technologies. AFE behavior in high‐k ZrO₂is of particular technological interest, but the origin of antiferroelectricity in ZrO₂remains questionable. The theory of reversible electric field‐induced phase transitions between the nonpolar P4₂/nmc tetragonal phase and the polarPca2₁orthorhombic phase is experimentally tested with local structural and electromechanical characterization of AFE ZrO₂thin films. Piezoresponse force microscopy identifies signature evidence of a field‐induced phase transition. A significant size effect in AFE ZrO₂is experimentally observed as film thickness is scaled down from 14.7 to 4.3 nm. The size effect is explained by modifications to the phase transition energy barrier heights ranging from 0.6 to 7.6 meV f.u⁻¹depending on crystallite size and in‐plane compressive strain with decreasing ZrO₂film thickness. Using the size effect, it is possible to double the energy storage density in ZrO₂from 20 J cm⁻³to greater than 40 J cm⁻³, thus highlighting a feasible route for superior performance in AFE fluorite supercapacitors.