Pb6Bi2Se9, the selenium analogue of he-yrovsyite, crystallizes in the orthorhombic space group Cmcm (#63) with a = 4.257(1) angstrom, b = 14.105(3) angstrom, and c = 32.412(7) angstrom at 300 K Its crystal structure consists of two NaCl-type layers, A and B, with equal thickness, N-1 = N-2 = 7, where N is the number of edge-sharing [Pb/Bi]Se-6 octahedra along the central diagonal. In the crystal structure, adjacent layers are arranged along the c-axis such that bridging bicapped trigonal prisms, PbSe8, are located on a pseudomirror plane parallel to (001). Therefore, Pb6Bi2Se9 corresponds to a L-7,L-7 member of the lillianite homologous series. Electronic transport measurements indicate that the compound is a heavily doped narrow band gap n-type semiconductor, with electrical conductivity and therrnopower values of 350 S/cm and -53 mu V/K at 300 K Interestingly, the compound exhibits a moderately low thermal conductivity, similar to 1.1 W/mK, in the whole temperature range, owing to its complex crystal structure, which enables strong phonon scattering at the twin boundaries between adjacent NaCl-type layers A and B. The dimensionless figure of merit, ZT, increases with temperature to 0.25 at 673 K.