Knowledge of the complex optical constants of the hybrid perovskite CsPbBr₃ in thin films is essential for their applications in optoelectronic devices. In this paper, we measured the optical constants of a perovskite CsPbBr₃ thin film deposited on an FTO substrate. We characterized the CsPbBr₃ thin film by X-ray diffraction, photoluminescence emission, atomic force microscopy, scanning electron microscopy, and absorption spectra. Complex optical constants of the CsPbBr₃ thin film were measured by spectroscopic ellipsometry and modeled by a Tauc–Lorentz formulation with seven oscillators. The observed interband transitions, which are positioned at 2.39, 3.44, and 3.85 eV, are denoted E_cp1, E_cp2 and E_cp3. We used critical point fitting of the second-derivative spectrum d²(E²ε)/dE² to analyze these spectral features. We assigned the origins of the three critical points and discuss their intrinsic mechanisms. These results indicate that interband transitions are governed by {PbBr₆}⁴⁻ octahedra, and small Cs⁺ ions affect the bandgap by breaking the octahedral packing symmetry. The simulated absorption coefficient agreed well with the experimental data.