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The solubility of niobium-94 (94Nb) in calcium alkaline solutions is one of the important parameters in safety assessment of intermediate-depth disposal which are assumed to use cementitious materials. Nb solubility and solubility-limiting solid phases of Nb in these systems remain unclear. The oversaturation solubility experiments were performed systematically in the 0.001–0.1 mol dm−3 (M) CaCl2 solutions under alkali conditions, and the characterization of precipitated solid phase controlling Nb solubility was conducted. The negative dependence of Nb solubilities on pH and calcium (Ca) concentration was observed in solubility experiments, and the Ca/Nb molar ratio of precipitated solid phase was 0.66. The pH and Ca dependence of Nb solubilities was reproduced by the reaction with Nb aqueous species Nb(OH)6 − and Ca–Nb oxide with the Ca/Nb ratio of 0.66, e.g., Ca4Nb6O19 (am). With increasing pH, Nb concentrations in the 0.001–0.1 M CaCl2 solutions were significantly lower than those calculated from thermodynamic data without Ca–Nb solid. This work provides systematic evidence that the presence of Ca clearly affects Nb solubility. Since calcium is a major component of groundwater and cement pore water, the Ca–Nb solid phase should be considered in the Nb solubility assessment.