High-strain-rate superplasticity (HSRS) is attained in tetragonal ZrO2 dispersed with 30vol% MgAl2O4. The tensile ductility exceeds 250% at 1823 K and even at a strain rate of approximate to0.7 s(-1). HSRS can be attained by the enhanced accommodation process of stress concentrations at multiple grain junctions exerted by grain boundary sliding. The present results indicate that the enhanced accommodation process occurs by the accelerated cation diffusion by the dissolution of Al and Mg ions from the MgAl2O4 phase into the ZrO2 matrix and by the accelerated stress relaxation through the plastic deformation of the MgAl2O4 grains.