Polycrystalline samples of Cu_3−xSn_xSe₃ were synthesized in the composition range x = 0.87–1.05. A compositionally induced evolvement from tetragonal via cubic to monoclinic crystal structures is observed, when the composition changes from a Cu-rich to a Sn-rich one. The Cu_3−xSn_xSe₃ materials show a metal-to-semiconductor transition with increasing x. Electronic transport properties are governed by the charge-carrier concentration which is well described by a linear dispersion-band model. The lattice component of the thermal conductivity is practically independent of x which is attributed to the opposite influence of the atomic ordering and the inhomogeneous distribution of the Cu–Se or Sn–Se bonds with different polarities in the crystal structure. The highest thermoelectric figure of merit ZT of 0.34 is achieved for x = 1.025 at 700 K.