Lead chalcogenide quaternary systems have been shown to provide high thermoelectric (TE) efficiency superior to those of binary andternary lead chalcogenides, arising from both altered electronic band structures and a reduction in lattice thermal conductivity. Here, we have synthesized single-phase samples of the quaternary compound (PbTe)0.55(PbS)0.1(PbSe)0.35 dopedwith Na and characterized their TE properties. We show that the dopant solubility is limited to 1 at. %. A very low lattice thermal conductivity of∼0.6 Wm−1K−1at 850 K is achieved at all dopant concentrations because of phonon scattering from point defects associated with solute atoms with high contrastatomic mass. As a result, a high TE figure of merit of approximately 1.5 is achieved at 823 K in heavily doped samples. Moreover, the figure of merit is greater than 1 over a wide temperature range above 675 K.