Five new semiconductors Pb5Sb12+xBi6−xSe32 (x = 0, 1, 2, 3, and 4) have been synthesized for the first time, which adopt pavonite-type structure and crystallize in monoclinic C2/m space group. The crystal structure is composed of two different types of polyhedral slabs. Slab-I is a galena-like structure motif that forms with [MSe6] (M = Pb, Sb, and Bi) octahedra and slab-II contains one octahedral [MSe6] block and paired squared pyramids [MSe5]. Pb5Sb12+xBi6−xSe32 exhibits n-type semiconductor behaviors and the remarkable Seebeck coefficient from −64.1 μV K−1 for x = 0 sample to −242 μV K−1 for x = 4 sample at 300 K. Moreover, the Pb5Sb12Bi6Se32 has the highest carrier concentration of 1.35 × 1020 cm−3 in pavonite-type materials. The complex compositions, mixed occupancies of the cations, and quasi-two-dimensional structure lead to the low lattice thermal conductivity (κlat) less than 0.48 W m−1 K−1 from 300 to 723 K, at which Pb5Sb16Bi2Se32 especially shows the ultralow value of 0.25 W m−1 K−1. As a result, the thermoelectric figure of merit, ZT ∼0.34 at 723 K, is obtained for the intrinsic Pb5Sb12Bi6Se32.