Ba3Mo18O28, a new member in the series Mn-xMo4n+206n+4, has been synthesized at 1230-degrees-C. Single crystals can be prepared through a transport reaction utilizing BaMoO4, MoO2, and Mo reagents in the presence of water vapor. Its properties are characterized via single-crystal and powder X-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. Ba3Mo18O28 crystallizes in the centrosymmetric monoclinic space group P2(1)/a, with cell parameters a = 9.939(2) angstrom, b = 9.377(2) angstrom, c = 13.057(2) angstrom, beta = 100.92(1)degrees, V = 1194.7(3) angstrom3, Z = 2, and R = 0.0427 (R(w) = 0.0509) for 1284 reflections with I > 3sigma(I). The compound contains Mo18O42 clusters with strong metal-metal bonding. These clusters consist of four trans edge-shared molybdenum octahedra bridged on all edges by oxygen atoms. A prominent feature of these tetrameric units is the extreme short-long-short arrangement of apical-apical molybdenum bond distances. The tetrameric clusters are interconnected in a stair-step fashion via Mo-O bonding. The sharing of oxygen atoms between clusters can be represented by the connectivity formula [(Mo18Oi14Oi-i4/2Oi-a12/2)Oa-i12/2]6-. Pockets created by the interconnected clusters are filled with barium atoms. Mo-Mo bond order and Mo-O bond valence sums indicate 61.7(6) and 58(2) electrons available for metal-metal bonding (MCE), respectively. The bond valence sums are in excellent agreement with the 58 MCE expected on the basis of formal oxidation states. This compound was found to have a large chi(TIP) contribution to the magnetic susceptibility. Resistivity measurements on a sintered, pressed pellet showed that the material is a small band gap semiconductor, as expected from the long (>3.17 angstrom) intercluster Mo-Mo distances.