Bytizite (Cu<sub>3</sub>SbSe<sub>3</sub> ) has attracted interest as a promising thermoelectric material because of its ultralow thermal conductivity; however, there are few experimental studies. This study investigated the optimal processing conditions for the synthesis of Cu<sub>3</sub>SbSe<sub>3</sub> using mechanical alloying (MA) and hot pressing (HP). The MA powder exhibited an orthorhombic Cu<sub>3</sub>SbSe<sub>3</sub> phase, which remained even after HP. However, secondary phases of permingeatite (Cu<sub>3</sub>SbSe<sub>4</sub>) and berzelianite (Cu<sub>1.78</sub>Se) were also identified in the X-ray diffraction patterns. Thermal analysis revealed that the MA powder and HP compacts exhibited a large endothermic peak near 727 K, which corresponds to the melting point of Cu<sub>3</sub>SbSe<sub>3</sub> . Dense compacts with a relative density higher than 99% were obtained at HP temperatures above 573 K. Microstructural and elemental analyses confirmed the presence of the secondary phase Cu<sub>3 </sub>SbSe<sub>4</sub> in the matrix of Cu<sub>3</sub>SbSe<sub>3</sub> . However, the Cu<sub>1.78</sub>Se phase could not be observed. All specimens exhibited an electrical conductivity of (0.66–1.06) × 10 3 Sm<sup>-1</sup>, a Seebeck coefficient of 324–376 µVK<sup>-1</sup>, and a power factor of 0.09–0.11 mWm<sup>-1</sup>K<sup>-2</sup> at 623 K. The thermal conductivity was lower than 0.7 Wm<sup>-1</sup>K<sup>-1</sup> in the measured temperature range, mainly due to the phonon scattering caused by the lone-pair electrons of Sb. A dip in thermal conductivity was observed at 423 K, which was possibly caused by the order-disorder transition of bytizite. The dimensionless figure of merit ZT increased with increasing temperature, and the maximum <i>ZT</i> was 0.16 at 623 K.