The solubility of sodium tungstate (Na₂WO₄) in a molten sodium hydroxide (NaOH) bath and its dependence on the partial pressure of water vapor were measured. This was done as part of an ongoing study of a new tungsten (W)-recycling process using molten NaOH. First, the nature of the chemical compound in the equilibrium solid phase in a molten NaOH bath was confirmed by adding excess amount of tungsten oxide as a tungstate ion source into the bath. X-ray diffraction analysis indicated that anhydrous Na₂WO₄ is the solid phase in equilibrium with the liquid phase of molten NaOH saturated with Na₂WO₄. Then, the solubility of the anhydrous Na₂WO₄ was measured under various partial pressures of water vapor. The results revealed that the solubility of anhydrous Na₂WO₄ drastically increased with the partial pressure of water vapor and the bath temperature from a molar fraction of 0.06–0.17. This value was much higher than those previously reported for molten nitrate and nitrites.