<p>This study presents a novel approach to enhance the photocatalytic performance of CeO₂ for tetracycline (TC) removal through controlled niobium (Nb) doping. Nb-doped CeO₂ nanoparticles (Nb1–CeO_2, Nb3–CeO_2, and Nb5–CeO₂) were synthesized via a hydrothermal method and characterized using XRD, FTIR, SEM, EDS, and TEM. XRD confirmed that the materials remained in an uncalcined cerium carbonate phase, with Nb5–CeO₂ showing mixed-phase features and greater hydroxyl functionalization, as revealed by FTIR. SEM and TEM analyses showed that Nb doping suppressed anisotropic growth, producing uniformly dispersed spherical nanoparticles with increased structural disorder. EDS mapping verified homogeneous Nb incorporation. Photocatalytic tests demonstrated that Nb5–CeO₂ achieved the highest TC degradation rate (<i>k</i> = 0.00705&#xa0;min⁻¹), with a normalized rate of 0.141&#xa0;min⁻¹&#xa0;g⁻¹, nearly double that of undoped CeO₂ (0.074&#xa0;min⁻¹&#xa0;g⁻¹), confirming the superior catalytic activity of the doped material. Scavenger experiments identified superoxide radicals (^•O₂⁻) and photogenerated holes (h⁺) as the dominant reactive species. Moreover, Nb5–CeO₂ maintained excellent stability over multiple reuse cycles. These findings establish Nb doping as a powerful strategy for engineering defect-rich, hydroxyl-functionalized CeO₂ photocatalysts and provide valuable insights for the development of recyclable, high-performance materials for wastewater treatment under light irradiation.</p>