The base catalytic activity of the decaniobate cluster (TMA)₆[Nb₁₀O₂₈]·6H₂O (TMA⁺ = tetramethylammonium cation) was studied theoretically and experimentally. Density functional theory calculations showed that the oxygen atoms in the cluster are highly negatively charged and suggested the possibility that the cluster can act as a base catalyst. We demonstrated for the first time that [Nb₁₀O₂₈]⁶⁻ actually exhibits base catalytic activity for aldol-type condensation reactions including Knoevenagel and Claisen–Schmidt condensation reactions. The catalytic reactions proceeded under ambient conditions, suggesting that [Nb₁₀O₂₈]⁶⁻ holds promise as a practical strong base catalyst.