Key phase diagram experiments were conducted to reveal the phase diagram of the K2O-MgO system using thermal analysis and equilibration techniques with sealed platinum capsules. The peritectic reaction of liquid + MgO → K6MgO4 was determined at 1131 K ± 14 K (858 °C ± 14 °C) for the first time. Based on the critical evaluation of all the literature data and the present experimental results, the thermodynamic optimizations of the K2O-MgO and K2O-MgO-SiO2 systems were performed to obtain a set of model parameters describing the Gibbs energies of all phases in the ternary systems. All reliable phase diagrams and thermodynamic properties of the K2O-MgO and K2O-MgO-SiO2 systems were well reproduced from the optimization. The optimized set of the Gibbs energies of all phases can be used to calculate any unexplored phase diagrams and thermodynamic properties in the system.