The reaction of [Bi2W22O74(OH)2]12− with Cu2+ in hot aqueous solution in the presence of Na+ affords the polyoxometalate salt Na12[Cu3Bi2W18O66]⋅32H2O. Single-crystal X-ray analysis [monoclinic, space-group C2/m, a = 33.082(6) Å, b = 19.615(5) Å, c = 14.885(2) Å, β = 90.632(12)°, Z = 4; R(F) = 0.065 for 6029 data with I>σ (I), wR(F 2) = 0.135 for all 8793 independent reflections] shows that the anion [{Cu(H2O)}3{BiW9O33}2]12−, on a crystal mirror-plane, has noncrystallographic symmetry close to D3h. Two B-α-[BiIIIW9O33]9− units are linked by an equatorial belt of three Cu(H2O)2+ units. Three Na(H2O)2 + units are also attached in the equatorial plane. The bond geometry is square-pyramidal about each Cu atom and trigonal–prismatic about each Na.