The effect of second phase dispersion on high-strain-rate superplasticity was examined in tetragonal ZrO₂ dispersed with 30 vol% MgAl₂O₄ spinel. The spinel particle enhances the diffusivity of ZrO₂ by supplying small amounts of aluminum and magnesium into ZrO₂ and suppresses grain growth by grain boundary pinning. After superplastic flow, the spinel particles highly elongate along the tensile direction. In the spinel particles, intragranular dislocations were observed, indicating that the spinel particles may contribute to the relaxation of stress concentrations around grain junctions exerted by grain boundary sliding. Comparison with earlier studies suggests that the dispersion of spinel particles can attain high-strain-rate superplasticity in tetragonal ZrO₂ through providing the following positive factors simultaneously; (i) the suppressed grain growth, enhanced accommodation process due to the accelerated (ii) diffusivity and (iii) stress relaxation.