TlGaS2 with a quasi-two-dimensional structure has been accessed by spectroscopic ellipsometry over the 1.5-6.0 eV spectral range. A uniaxial approach applicable to monoclinic TlGaS2 at room temperature has been employed for ellipsometric data treatment. Principal components of the dielectric function tensor have then been retrieved. Inter-band optical transitions associated with the obtained dielectric function have been determined by using standard critical point analysis. The transitions have been assigned within the electronic band structure obtained for TlGaS2 from calculations based on density functional theory. (C) 2014 Elsevier B.V. All rights reserved.