A novel diphosphoheptadecatungstate complex, [(P2O7)W17O51]4− was prepared from a 200 mM (M = mol dm−3) W(VI)–10 mM P2O74−–0.6 M HCl–40% (v/v) CH3CN system, and characterized by IR, UV–vis, 31P NMR, and 183W NMR spectroscopy, and voltammetry. Single-crystal X-ray analysis was made on [(C2H5)4N]4[(P2O7)W17O51]·CH3CN, which crystallized in the monoclinic space group P21/n, with cell parameters a = 16.1877(8), b = 22.103(1), c = 22.670(1) Å, β = 90.847(1)°, V = 8110.3(7) Å3, and Z = 4. The anion consists of a PW8O31 fragment derived from a β-Keggin-type [PW12O40]3− structure, and two PW8O31 units are linked by five oxygen atoms and a WO5 square pyramid. The [(P2O7)W17O51]4− anion exhibited a four-step reversible one-electron redox wave in CH3CN, and a three-step two-electron redox wave resulted in the presence of acid. This is the first example of an electrochemically-active diphosphotungstate complex.