<p>In this study, a novel Gd-doped CdZnS nanocomposite was successfully synthesized via a simple aqueous co-precipitation method and evaluated for its structural, optical, and photocatalytic properties. The incorporation of Gd³⁺ ions into the CdZnS lattice significantly influenced the crystal structure, morphology, and defect density, as confirmed by XRD, Raman, and SEM analyses. The substitution of Cd²⁺ by Gd³⁺ induced compressive lattice strain, reduced crystallite size, and promoted defect-rich surface structures. Optical characterizations revealed a red-shifted absorption edge and Narrowed band gap from 2.16 to 2.01&#xa0;eV upon doping, attributed to the formation of localized states and enhanced electron–phonon interactions. Photoluminescence (PL) spectroscopy further supported the presence of defect-assisted recombination and Gd-related energy levels. Most notably, the Gd-doped CdZnS exhibited superior photocatalytic efficiency, achieving 98% degradation of Rhodamine B under visible Light within 60&#xa0;min-surpassing undoped and binary counterparts. Kinetic studies yielded a moderate activation energy of 29.77&#xa0;kJ/mol, confirming thermally activated behavior. The catalyst also retained &gt; 85% activity after five reuse cycles, demonstrating excellent stability. The synergistic effects of Gd doping on band structure modulation, defect engineering, and reusability underline the potential of Gd-doped CdZnS as a high-performance, reusable photocatalyst for environmental remediation applications.</p>