<p>The increasing contamination of water bodies by organic dyes and industrial effluents has created an urgent need for the development of new, cost-effective, and highly efficient materials capable of degrading pollutants in wastewater. In this work, undoped and Na-doped Cu₂MgSnS₄ (CMTS) thin films were synthesized via spray pyrolysis on glass substrates. The structural, optical, and photocatalytic properties of the films were systematically investigated. X-ray diffraction (XRD) analysis revealed enhanced crystallinity and increased grain size upon Na incorporation into the CMTS lattice. Both XRD and Raman spectroscopy confirmed the polycrystalline kesterite structure with a preferred (112) orientation. FTIR analysis further supported the successful incorporation of Na⁺ ions, with observable shifts and intensity changes in the metal–sulfur (M–S) vibrational bands, particularly in the 600–650 cm⁻¹ range, indicating lattice distortion due to doping. Additional features corresponding to hydroxyl and adsorbed water groups were also detected, which may influence photocatalytic interactions. Optical studies indicated increased absorbance and reduced transmittance in doped films, with the 7.5% Na-doped CMTS showing an optimized bandgap of 1.54 eV. The photocatalytic activity of the films was assessed by degrading two common organic pollutants, Malachite Green and Rhodamine B, under visible light irradiation. At optimal doping levels (5% and 7.5% Na), degradation efficiencies reached 90% and 95%, respectively. Furthermore, the effects of photocatalyst dosage, the presence of scavenger agents, and solution pH were thoroughly examined to optimize the degradation process. These findings highlight the potential of Na-doped CMTS thin films as promising photocatalysts for environmental remediation applications, offering a sustainable route for wastewater treatment.</p>