Experimental observations of the tetragonal phase transformation behavior in polycrystalline zirconias and the related toughening contribution are presented. An analysis which considers transformation thermodynamics and residual stresses is developed to describe the influence of grain size on tetragonal‐to‐monoclinic transformation temperature. The model is based on the promotion of the transformation by local internal tensile stress concentrations whose effects scale with grain size. The analysis is supported by observations of the martensite start temperature–grain size behavior in polycrystalline tetragonal zirconia containing 12 mol% ceria (12CeTZP). Next, the analysis considers the grain‐size‐dependent behavior of the transformation‐toughening contribution, ΔKT, and the transformation zone size,rT. The tetragonal‐to‐monoclinic (t‐to‐m) formation temperature,Ms, increases with increase in tetragonal grain size,d. Experimental results for zirconia‐12 mol% ceria (12CeTZP) and 2YTZP ceramics illustrate the predicted forms of the grain size dependencies for ΔKTandrT. The analytical model also describes the temperature dependence of the transformation‐toughening contribution ΔKTobserved in 2 mol% yttriadoped polycrystalline te