A structurally intriguing polyanion, [{Nb4O6(OH)(4)}{Nb6P2W12O61}(4)](36)- (2), was directly formed by controlling the reaction parameters and characterized by single-crystal X-ray diffraction, IR spectroscopy, and elemental analyses. Polyanion 2 is the first niobium-substituted WellsDawson-based tetramer and contains the largest Nb-28 cluster encapsulated by four hexalacunary WellsDawson frameworks. The activities of compounds Na-12[H-24-(2)].24H2O (2a), K-12[H2P2W12O48].24H(2)O, and K-7[HNb6O19].13H(2)O against three human tumor cell lines were investigated in vitro. The preliminary results revealed that 2a could efficiently inhibit the growth of the human breast cancer MCF-7 cells. Interestingly, the activity of 2a is significantly better than those of the simple reagents.