Natural and synthetic tetrahedrites are currently the focus of attention in thermoelectricity due to their abundance, numerous possibilities of substitution, and their complex crystal structure () resulting in extremely low lattice thermal conductivity (below 1 W m⁻¹ K⁻¹ above 300 K). Here, we report on the synthesis, structural and chemical characterizations, and high-temperature thermoelectric properties of polycrystalline Cu_12−x M _x Sb_4−y Te _y S₁₃ (M = Zn, Ni) tetrahedrites. Upon substituting on both the Cu and Sb sites, we successfully improved the thermoelectric properties with respect to the ternary compound Cu₁₂Sb₄S₁₃ with peak ZT values of 0.7 at 700 K.