We report first-principles-based calculations, combined with an efficient Monte Carlo technique, that undoubtedly show that Pb(Zr0.5Ti0.5)O3, one of the most important ferroelectrics to date, adopts critical behavior that strongly deviates from the classical mean-field approach while being, in fact, consistent with the 3D-random Ising universality class. © 2009 The American Physical Society.