Ferroelectric materials are highly promising for next-generation electro–optic (EO) modulators because of their ultrafast and efficient light modulation. However, efforts to maximize polarization freedom for large refractive index modulation—through domain engineering, epitaxial strain, and defect engineering—have hit limitations, leaving intrinsic polarization mechanisms largely unexplored. Here, we report a giant effective EO coefficient (~233.5 pm/V) in PbZr_0.52Ti_0.48O₃ (PZT) films, which surpasses all reported values measured under an in-plane electric field and significantly exceeds the theoretical limit (~13 pm/V) as well as the value of LiNbO₃ (~31 pm/V). Beyond conventional domain switching, phase transitions and domain wall variations critically enhance the EO effect. The highly relaxed structure of the PZT film, with mixed [001] and [100] orientations and disordered nanoscale phases, enables unprecedented polarization control. This unique configuration breaks the theoretical EO coefficient limit, bridging the gap between predictions and experimental results. Owing to its high Curie temperature and compatibility with wafer-scale fabrication, PZT has emerged as a promising candidate for next-generation high-performance EO modulators. Our findings not only advance the frontiers of ferroelectric EO materials but also pave the way for exploring other ferroelectric thin-film devices, such as those for energy storage and electrocaloric cooling, by leveraging enhanced polarization modulation mechanisms.