<p>This study systematically investigates the influence of configurational entropy on the phase structure, microstructure, defect chemistry, and dielectric properties of four A-site disordered perovskite ceramics (Ba, La)TiO₃, (Ba, La, Sr)TiO₃, (Ba, La, Sr, Ca)TiO₃, and (Ba, La, Sr, Ca, Bi)TiO₃. X-ray diffraction analysis indicates that all compositions exhibit a single perovskite structure. With increasing configurational entropy, the diffraction peaks systematically shift to higher angles. Rietveld refinement confirms a reduction in unit cell volume, providing direct evidence of lattice contraction. Scanning electron microscopy suggests that the high-entropy sample exhibits a smaller average grain size compared to the low-entropy counterpart. This reduction is attributed to the hysteresis diffusion effect and suppressed grain boundary migration arising from lattice distortion. X-ray photoelectron spectroscopy analysis shows that high-entropy conditions promote the partial reduction of Ti⁴⁺ to Ti³⁺ and the increased concentration of oxygen vacancies, forming defect dipoles through the charge compensation mechanism. Dielectric measurements reveal that the (Ba, La, Sr, Ca, Bi)TiO₃ composition exhibits a dielectric constant of 3.06 × 10⁵ at 100&#xa0;Hz, twice that of (Ba, La)TiO₃, and a dielectric loss as low as 0.004 in the high-frequency regime. Impedance spectroscopy verifies that the internal barrier layer capacitance mechanism dominates the dielectric response, with an activation energy of 0.52–0.63&#xa0;eV corresponding to thermally activated migration of oxygen vacancies. We demonstrate that configurational entropy engineering achieves an optimized balance between high dielectric constant and low dielectric loss by synergistically regulating lattice strain, microstructure, and defect chemistry, providing a new strategy for advanced dielectric materials.</p>