•The (100) and (001) surfaces of TlGaSe2 are accessed by spectroscopic ellipsometry.•Polarization-and temperature-dependent dielectric function spectra are obtained.•Critical points for inter-band optical transitions are retrieved.•Optical transitions are assigned according to the calculated band structure.•Critical points associated with Tl-orbitals show remarkable temperature behaviour. The dielectric function spectra of TlGaSe2 crystal with quasi-two-dimensional layered structure were studied over the photon range 1.5–5.0eV in the temperature range 80–400K. The (100) and (001) surfaces were accessed by spectroscopic ellipsometry and the dielectric function was retrieved after conventional treatment of the ellipsometric data. Inter-band optical transitions associated with the obtained dielectric function were determined by using standard critical point analysis. Assignment of the transitions was done within the framework of the calculated electronic band structure. An abrupt temperature-induced change in energy of the retrieved critical points for inter-band optical transitions between the electronic states formed with participation of thallium orbitals was obtained at 104.5, 123.3 and 247.0K. The last temperatures agree well with the temperature points of the subsequent structural phase transitions in TlGaSe2.