Abstract Engineering materials with highly tunable physical properties in response to external stimuli is a cornerstone strategy for advancing energy technology. Among various approaches, engineering ionic defects and understanding their roles are essential in tailoring emergent material properties and functionalities. Here, we demonstrate an effective approach for creating and controlling ionic defects (oxygen vacancies) in epitaxial Gd-doped CeO_2−x (CGO)(001) films grown on Nb:SrTiO₃(001) single crystal. Our results exhibit a significant limitation in the formation of excess oxygen vacancies in the films during high-temperature film growth. However, we have discovered that managing the oxygen vacancies in the epitaxial CGO(001) films is feasible using a two-step film growth process. Subsequently, our findings show that manipulating excess oxygen vacancies is a key to the emergence of giant apparent dielectric permittivity (e.g. ε ′ ≈ 10⁶) in the epitaxial films under electrical field control. Overall, the strategy of tuning functional ionic defects in CGO and similar oxides is beneficial for various applications such as electromechanical, sensing, and energy storage applications.