<p>Tunable size cerium oxide nanoparticles doped with varying concentrations of Cd²⁺ ions are prepared using the hydrothermal technique. The powder XRD and EDS analyses confirmed the inclusion of Cd²⁺ ions at the Ce³⁺/Ce⁴⁺ position while maintaining the fluorite cubic structure. The typical crystallite size diminishes as the Cd²⁺ dopant concentration is increased. The electron microscopy reveals that Cd²⁺ doped CeO₂ nanoparticles exhibits a spherical shape, and the particle size diminishes with higher concentration of Cd²⁺ ions. The EDS studies have confirmed the existence of Ce, Cd, and O in the synthesized samples. The band gap energy of Cd²⁺ doped CeO₂ nanoparticles rises from 3.55 to 4.03&#xa0;eV as the dopant concentration increases from 5 to 15&#xa0;M%. The observed rise in the bandgap energy of Cd²⁺ doped CeO₂ nanoparticles may result from the reduction in particle size. The Raman F2g peak is diminished as strain, oxygen vacancy concentration and the number of Ce–O vibrations decrease with rising Cd²⁺ dopant concentration levels. The dielectric studies indicate that at low frequencies, the dielectric constant for all samples declines exponentially with rising frequency, while at higher frequencies, the dielectric constant for all samples remains nearly constant. As the concentration of Cd²⁺ dopant rises, the dielectric constant’s value diminishes, whereas the conductivity’s value escalates. These changes may be attributed to the oxygen vacancy defects created by the substitution of Cd²⁺ ions on Ce⁴⁺/Ce³⁺ ions. The Photocatalytic study reveals that the methylene blue degradation efficiency of Cd²⁺ doped CeO₂ nanoparticles increases as Cd²⁺ dopant ions concentration increases (5–15&#xa0;M%).</p>