The monoclinic structure TlIn_1-xTm_xS₂ was studied based on the density functional theory (DFT). The cases of substitution of indium atoms with thulium are considered. The lattice cell parameters were determined in optimized supercells TlInS₂ taking into account the approximation of the local density. Lattice constants for a layered crystal TlIn_1-xTm_xS₂ were theoretically determined based on calculations of a monoclinic structure with a space group C/2c (coordination number Z=16, N 15) and compared with experimental results. New semiconductor polycrystals of TlIn_1-xTm_xS₂ (x=0, 0.001 and 0.005) compositions were synthesized in quartz ampoules and corresponding single crystals were grown from them by directional solidification method. Analysis of X-ray diffraction patterns shows that all compositions TlIn_1-xTm_xS₂ have stable monoclinic crystal system with space group C/2c. The calculated parameters of the lattice cell of TlIn_1-xTm_xS₂ samples confirm this. Dielectric properties in alternating electric fields with a frequency of f=5· 10^4-3.5·10⁷ Hz at room temperature were studied in single crystals. The relaxation character of the dielectric constant, the nature of dielectric losses, and the hopping mechanism of charge transfer in TlIn_1-xTm_xS₂ samples were determined. The parameters of localized states in crystals of TlIn_1-xTm_xS₂ samples were calculated using the Mott model. It was shown that, the AC conductivity, the density of localized states near the Fermi level, the average distance and time of charge carrier hops in TlIn_1-xTm_xS₂ increase compared with the undoped TlInS₂. Keywords: monoclinic structure, DFT LDA, single crystal TlInS₂, doping impact, thulium impurity, lattice cell parameters, permittivity, hopping conductance, frequency dispersion, dielectric losses.