In this work, optical properties of undoped zinc oxide (ZnO) and chromium (Cr) doped ZnO prepared at different concentrations of Cr (2, 3, and 5 wt.%) on glass substrates by a spray pyrolysis method are reported. The structural properties investigated by X-ray diffraction revealed the hexagonal wurtzite structure, noting that the crystallite size of the films decreases with increasing Cr content. The optical characterization of the samples was carried out using spectral transmittance. The refractive index, energy gap, and extinction coefficient of pure and Cr-doped ZnO thin films have been calculated. The single oscillator model of Wemple–DiDomenico was used to study the dispersion of the refractive index. The oscillator parameters, the single oscillator energy, the dispersion energy, and the static refractive index were determined. The linear optical susceptibility and non-linear optical susceptibility were also studied and discussed. These promising results achieved by Cr-doping of ZnO exhibited an important behavior for technological applications in electronic, optoelectronic devices and non-linear optical applications.