A new mixed-valence vanadium diphosphate, KV3P4O16, was synthesized by solid-state reaction and its structure was determined from single-crystal X-ray diffraction data. Brown crystal of KV3P4O16 crystallizes in the monoclinic space group P21/m (No. 11) with a = 5.201(1), b = 12.661(2), c = 9.476(2) Å, β = 94.11(2)°, V = 622.4(2) Å3, Z = 2, R = 0.0293, Rw = 0.0333 for 1108 unique reflections with I > 2.5 σ(I). Edge-sharing pairs of VO6 octahedra are connected by sharing corners to form strings along the b axis. The strings formally contain V3.5+ (one V3+ and one V4+) and are connected through P2O7 groups such that layers in the ab plane are formed. Adjacent layers are linked in three dimensions by VO2+ vanadyl groups. The structure consists of tunnels where the K+ cations are located. The structural formula is K(V2O)(VO)(P2O7)2. Variable-temperature powder magnetic susceptibility data confirm the presence of one V3+ and two V4+ ions per formula unit.