Electronic structure, DOS effective mass, conductivity effective mass, reduced effective mass are calculated for a TlInS$_{2}$ crystal from first principles within the framework of the density functional theory. The electronic structure of TlInS$_{2}$ is investigated by the Quantum Wise and Quantum Espresso software program packages within the LDA and SGGA corrections, respectively. The band structures calculated with use of the FHI exchange-correlation and relativistic pseudopotentials show that both the valence band maximum and the conduction band minimum located at the $\Gamma$ point of the Brillouin zone. The top of the valence band and the bottom of the conduction band originate generally from 6s-states of Tl atoms and 5s-states of In atoms. Influence of the spin—orbit interaction on the electronic structure is investigated. As found, inclusion of the spin—orbit interaction is not essential for the energy states near the gap and removes the band degeneracy along the T—Z symmetry line, but does not remove the degeneracy at the T, Z, and $\Gamma$, Y points. The maximum value of the spin—orbit splitting is $\cong$ 0.3 eV. The effective masses along [001], [010], and [100] directions are calculated taking into account non-parabolicity of the bands. In this paper, DOS effective mass, conductivity effective mass, and reduced effective mass are calculated theoretically for the first time.