Nickel selenide thin films were grown on glass substrates at room temperature using the successive ionic layer adsorption and reaction (SILAR) method. Optical and structural analyses of the thin films were also performed. X-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX) and scanning electron microscopy (SEM) were used for structural analysis, and UV–vis spectrometer was used for optical analysis. The XRD results indicate that the NiSe2 thin films had a polycrystalline structure. As a result of the optical analysis, the bandgap value decreased from 2.56 to 2.25 eV as the thickness increased. These results suggest that increasing film thickness improves the crystal structure of NiSe2 thin films. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.