<p>Accurate prediction of the effective dielectric constant in polymer nanocomposites requires explicit consideration of the interphase region between ceramic fillers and the polymer matrix, a factor commonly neglected in classical effective medium theories. In this study, we develop an interphase-aware modeling framework by extending established models (e.g., Maxwell–Garnett, Poon–Shin, and Bergman) through a core–shell representation of nanoparticles. The proposed models are validated against experimental data for two representative systems: lead-containing PZT–PVDF and lead-free KNN–PAN nanocomposites. Results demonstrate that incorporating the interphase significantly improves prediction accuracy, reducing the coefficient of variation by up to 50% compared to original models. For instance, the modified Poon–Shin model achieves a deviation of only 24% (vs. 30% for the original) for PZT–PVDF, while the enhanced Palletto model yields just 6% deviation for KNN–PAN (vs. 4% for the original, with better trend capture). Parametric studies further reveal that the effective dielectric constant increases with interphase thickness and filler content but decreases with nanoparticle radius, trends consistent with interfacial polarization mechanisms. This work provides a robust, generalizable approach for modeling high-performance, eco-friendly dielectric nanocomposites, with direct relevance to next-generation energy storage applications.</p>