<p>This study aims to investigate the structural, morphological, optical, and photocatalytic properties of MgO and Cu-doped MgO thin films with Cu concentrations of 3, 6, and 9 at.% deposited on glass substrates via the sol–gel spin-coating technique. The effects of Cu doping on these properties of the thin films were investigated using scanning electron microscopy (SEM), energy‑dispersive X‑ray spectroscopy (EDX), X‑ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and UV–Vis spectrophotometry. The structural analysis indicated that the deposited films exhibited a cubic crystal structure without secondary phases, with (2 0 0) preferred orientation. Crystallite size decreased from 31.25 nm to 15.48 nm with the introduction of dopants. Additionally, the SEM images revealed that doping with Cu switched the nanowall structures to rounded. The surface roughness tends to decrease with Cu-doping, as observed from the AFM topographical image. Optical characterization indicated a redshift in the absorption edge and a decrease in band gap energy from 3.93 eV to 3.82 eV based on Tauc plots for direct transitions (αhν)² vs hν, alongside increased refractive index (n), extinction coefficient (k), and dielectric constant (ε), increased with Cu content, particularly in the visible region. Photoluminescence (PL) intensity diminished with rising Cu content, suggesting suppression of radiative recombination. Finally, photocatalytic experiments using methylene blue (MB) under natural sunlight showed that 6% Cu-doped MgO films achieved the highest degradation efficiency, exceeding 97% within 200 min. These findings demonstrate that Cu doping effectively tunes the optical and catalytic behavior of MgO thin films, making them promising candidates for solar-driven environmental remediation and optoelectronic applications.</p><p></p>