Pb7Bi4Se13 crystallizes in the monoclinic space group C2/m (No. 12) with a = 13.991(3) angstrom, b = 4.262(2) angstrom, c = 23.432(5) angstrom, and beta = 98.3(3)degrees at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses N-1 = 5 and N-2 = 4 [N = number of edge-sharing (Pb/Bi)Se-6 octahedra along the central diagonal] are arranged along the c axis in such a way that the bridging monocapped trigonal prisms, PbSe7, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (similar to 0.33 W m(-1) K-1 at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb7Bi4Se13 is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb7Bi4Se13 is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (similar to-160 mu V K-1 at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb7Bi4Se13 and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples.