We present first-principles calculation of lattice dynamics of the TlGaSe2 ternary semiconductor compounds. Calculations have been performed using open-source code ABINIT on the basis of the density functional perturbation theory within the plane-wave pseudopotential approach. Results on the frequencies of phonon modes in the center of the Brillouin zone and the dispersion of transverse shear acoustic branch of the phonon spectra agree well with experimental data on Raman scattering, infrared reflectivity, and ultrasound wave propagation in TlGaSe2. The calculated and experimental temperature dependences of heat capacity are in a good agreement up to the room temperature. Along the layer, the low-frequency acoustic branch displays the bending wave behavior which is characteristic of the layer crystal structures. We have calculated elastic and compliances tensor, the directional Grüneisen functions, and the coefficients of the linear thermal expansion TlGaSe2. We obtained that both principal coefficients of linear thermal expansion are positive in the range of temperature above 5 K.