<p>Chalcostibite CuSbS₂ has attracted attention as an environmentally friendly material in thermoelectric (TE) field. Our investigations into the TE properties of high-quality p-type CuSbS₂ single crystals revealed that they exhibit a uniquely low thermal conductivity caused by the active lone-pair electrons in Sb³⁺ ions. The electrical conductivity was improved by the high density of shallow acceptor Cu vacancies and the absence of potential barriers to carrier transport. Consequently, the figure of merit for the Cu-poor CuSbS₂ single crystal reached 0.5 at 700 K, which is 25 times higher than that of the reported polycrystalline sample.</p>