The optical properties of Tl2InGaS4 layered single crystals have been studied by means of transmission and reflection measurements in the wavelength range of 400-1100 nm. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.35 and 2.54 eV, respectively. Transmission measurements carried out in the temperature range of 10-300 K revealed that the rate of change of the indirect band gap with temperature is gamma =-4.70 x 10(-4) eVK(-1). The absolute zero value of the band gap energy was obtained as E-gi( 0) = 2.45 eV. The 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 5.73 eV, 31.46 eV, 11.72 x 10(13) m(-2) and 2.55, respectively. From x-ray powder diffraction study, the parameters of the monoclinic unit cell were determined.