In this study, Co-doped ZnO thin film was deposited by spray pyrolysis technique at 230 ^∘C followed by annealing at 350 ^∘C for 1 h. Crystal structure was investigated by X-ray diffraction method. All the samples showed polycrystalline hexagonal wurtzite structure. Variation of lattice parameters, volume of unit cell, and Zn–O bond length were investigated. The surface morphology of the films was studied by SEM measurements and wrinkled surface were observed for the Co-doped ZnO films. The optical characterization of the annealed samples was performed. The observed band gap values were found to decrease on Co doping in ZnO thin films. The effect of Co substitution on important optoelectronic properties, such as variation of the extinction and absorption coefficient, refractive index, and dielectric constant of the ZnO thin films were thoroughly investigated. It was found that Co doping has significant effect on these properties over a certain range of wavelength.