A new, quaternary mercury and cadmium pnictidehalide semiconductor (Hg6Sb4)(CdI6) (1) has been prepared by the solid-state reaction and structurally characterized by single-crystal X-ray diffraction analysis. Compound 1 crystallizes in the space group R-3c of the rhombohedral system with four formula units in a cell: a=13.3818(9) A, alpha=90.93 degrees, and V=2395.3(3) A3. The crystal structural novelty of 1 derives from the fact that a 3-D mercury antimonide cationic network interpenetrates with an unusual 3-D cadmium iodide octahedral anionic network through the weak covalent interactions between mercury and iodine atoms to form a mixed 3-D framework. Among them, the cationic moiety features a perovskite-like 3-D network while the anionic moiety is characterized by a 3-D I6 octahedral anionic network with two-thirds I6 octahedra being occupied by Cd atoms. The optical properties were investigated in terms of the diffuse reflectance and Fourier transform infrared spectra. The electronic band structure along with density of states calculated by the density functional theory method indicates that compound 1 is a semiconductor with a direct band gap and that the optical absorption of 1 is mainly ascribed to the charge transitions from I-5p and Sb-5p states to Cd-5s and Hg-6s states.