A novel mixed-valence vanadium pyrophosphate, RbV3P4O17+x (x = 0.14), was discovered, and its structure was determined from single-crystal X-ray diffraction data. This nonstoichiometric phase crystallizes in the tetragonal space group P42/mnm with a = 13.651 (2) Å, c = 7.289 (2) Å, V = 1358.3 (4) Å3, Z = 4, R = 0.0366, and Rw = 0.0360 for 835 unique reflections with I > 2.5σ(I). The structure may be regarded as built from ReO3-type infinite chains along the c axis (in which each VO6 octahedron shares two opposite vertices) and finite chains parallel to the (110) directions, which are linked by P2O7 groups to form a three-dimensional structure. Each finite chain consists of four V06 subunits. The oxygen atom shared between the central and side octahedra in the four-membered chain is about two-thirds occupied. Variable-temperature powder magnetic susceptibility data suggest the presence of two V4+ and one V5+ per formula unit and support the formula determined from single-crystal X-ray diffraction data.