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In this paper, the authors report first‐principles calculations of the Seebeck coefficient in TlInSe2. Experimental measurements demonstrate that TlInSe2 is characterized by giant values of the Seebeck coefficient at the temperature region of 320–430 K. To get insight on significant enhancement of Seebeck coefficient under temperature changing the density functional theory (DFT) and the Boltzmann transport equation are applied to calculate the semiclassical transport coefficients for TlInSe2 and its constructed models. The molecular dynamic simulations of 2 × 2 × 2 and 1 × 1 × 8 superlattices of TlInSe2, the TlInSe2_118a, TlInSe2_118b, TlInSe2_222a, TlInSe2_222b structures are performed to study their behavior in both "bulk" and "chain – like" modes. It is found that the distortions of InSe4 tetrahedra and displacements of Tl atom lead to formation of Tl–Se covalent bonds. The "bulk" TlInSe2_222b compound possesses the maximum value of Seebeck coefficient and (ZT)max in comparison with another studied compounds. The hybridization of Se and Tl orbitals near the top of valence band and hybridization of the Se, Tl, and In orbitals in conduction band may lead to increasing of Seebeck coefficient in TlInSe2 at 425 K. [ABSTRACT FROM AUTHOR]