<p>Indium-doped cadmium sulfide (CdS:In) thin films are important, especially for applications in solar cells. Optimization of optical properties is very important for high device performance, and film thickness is a significant factor affecting the optical parameters. In this work, thin films of CdS:In were deposited by spray pyrolysis (SP) technique on glass substrates at substrate temperature <i>T</i>_<i>s</i> = 490°C. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), and ultraviolet-visible (UV-vis) spectroscopy. The films presented a polycrystalline structure and mixed (cubic and hexagonal) phase. The transmittance of the films was measured at room temperature and used to deduce the optical parameters: absorption coefficient, extinction coefficient, refractive index, the real and imaginary parts of the dielectric constant, and the energy loss. A redshift of the absorption edge, which increases with film thickness, was observed. It was found that the extinction coefficient and energy loss decrease with film thickness, while the refractive index increases with film thickness, and this increase becomes faster with decreasing wavelength of incident light. The real and imaginary parts of the dielectric constant showed fluctuations with film thickness, depending on interference maxima and minima in the transmittance spectra.</p>