A coupled experimental phase diagram study and thermodynamic modeling of the Li2O-Na2O-SiO2 system was conducted at 1 atm total pressure. Differential scanning calorimetry (DSC) measurements and equilibrium/quenching experiments along the Na2SiO3-Li4SiO4 isopleth were conducted using sealed Pt or Au-Pd crucibles to obtain the phase diagram data in the alkali-rich region. It is found that Li3NaSiO4 is the only compound present in the ternary system. Using all available and reliable phase diagram data in the literature and the present new phase diagram results, the thermodynamic optimization of the Li2O-Na2O-SiO2 system was carried out to obtain a set of Gibbs energy functions for the liquid phase and all solid phases to reproduce the phase equilibria of the present system.