The optical properties of Tl2In2Se3S layered single crystals have been analyzed using transmission and reflection measurements in the wavelength region between 500 and 1100 nm. The optical indirect transitions with a band gap energy of 1.96 eV and direct transitions with a band gap energy of 2.16 eV were determined from analysis of absorption data at room temperature. Dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters - oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index - were found to be 4.67 eV, 45.35 eV, 1.38 x 1014 m-2 and 3.27, respectively. Transmission measurements were also performed in the temperature range 10-300 K. As a result of temperature-dependent transmission measurements, the rate of change in the indirect band gap with temperature, i.e. = -5.6 x 10-4 eV/K, and the absolute zero value of the band gap energy, Egi(0) = 2.09 eV, were obtained.