A complete literature review, critical evaluation, and thermodynamic optimization of experimental phase diagrams and thermodynamic properties of the Na2O-FeO-Fe2O3 system were performed at 1 bar total pressure. A set of optimized model parameters obtained for all phases present in this system reproduces available and reliable thermodynamic properties and phase equilibria within experimental error limits from 298 K (25 °C) to above liquidus temperatures for all compositions and oxygen partial pressures from metallic saturation to 1 atm. The liquid phase was modeled based on the Modified Quasichemical Model by considering the possible formation of NaFeO2 associate in the liquid state. Complicated subsolidus phase relations depending on the oxygen partial pressure and temperature were elucidated, and discrepancies among experimental data were resolved.