In this paper we present the results of ab initio calculations of the lattice dynamics of ternary TlGaS2 semiconductor with highly anisotropic crystal structure. Calculations have been performed using the open-source code ABINIT on the basis of density functional theory within the plane-wave pseudopotential approach. The results are discussed in terms of previously existing experimental data. Calculated frequencies of the phonon modes at the center of the Brillouin zone, as well as calculated IR reflection spectra in the range of 15–500 cm−1, are in reasonable agreement with existing experimental data. The theoretical temperature dependence (up to room temperature) of the heat capacity is in good agreement with experimental data.