A new reduced rubidium niobium oxide, Rb1.51Nb10O17, was prepared by high temperature reaction of RbNbO3 and Nb in an evacuated silica tube. The niobium oxide crystallized in the trigonal system with a = 6.020(1) and c = 38.703(8)Angstrom A, space group R ($) over bar 3m, and Z = 3. The crystal structure was determined by single crystal X-ray diffraction analysis, and the final R factors were R = 0.053 and Rw = 0.064 for 352 unique reflections. There are three different types of niobium in this oxide. The reduced niobium is in octahedral [Nb6O12]O-6 clusters which form layers perpendicular to the c-axis. In addition, there are two types of fully oxidized niobium : Nb(2) octahedrally coordinated by six oxygens and Nb(3) tetrahedrally coordinated by four oxygens. The Nb(2)O-6 octahedra and Nb(3)O-4 tetrahedra are linked by corner-sharing and are sandwiched between the [Nb6O12]O-6 cluster layers. Rubidium is randomly distributed in a site coordinated by nine oxygens in the region of the Nb(2)O-6 octahedra and Nb(3)O-4 tetrahedra. Evidence for superconductivity was sought, but not found, down to 4.2 K.