This paper reports the influence of silver (Ag) addition on the optical constants of chalcogenide (Se80Te20)100−xAgx (0 ≤ x ≤ 4) thin films. Films of (Se80Te20)100−xAgx (0 ≤ x ≤ 4) glasses were prepared by thermal evaporation of the bulk samples. The transmittance and reflection spectra of amorphous thin films were obtained in the spectral region in the range 400–2500 nm. The optical band gap and the absorption coefficient were calculated from the transmission and reflection data. Swanepoel method has been applied to derive the real and imaginary parts of the complex index of refraction, refractive index, and also the thickness of amorphous films. The dispersion parameters are calculated by using the Wemple–DiDomenico model. The optical constants, i.e., the refractive index, extinction coefficient, absorption coefficient, and optical band gap are determined and their compositional dependence is considered. The results are discussed on the basis of defects and disorders present in chalcogenide systems.