Icosaniobate [Nb20O54](8-) was synthesized by reacting [H4Nb6O19](4-) with NO in tetrahydrofuran or MeNO2. A single-crystal X-ray diffraction study of its n-tetrabutylammonium salt [a = 17.7284(18) angstrom, b = 33.542(3) angstrom, c = 34.316(2) angstrom, Z = 4, and space group P22(1)2(1)] revealed a dimeric structure where two decaniobate ions are condensed sharing two terminal O atoms. Unlike that in [(NbW5O18)(2)O](4-), the Nb-O-Nb bridges in icosaniobate are bent. The nonlinear bridging reduces the maximum possible symmetry of the dimeric anion to mm 2, which it closely approximates.