Solid-state phase equilibria in the MgO-Y2O3-ZrO2 system as well as the equilibria including liquid were investigated in the whole-compositional range using high-temperature differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). Isothermal sections at 1493, 1573, 1693, and 1923 K were constructed based on experimental studies. The presence of tie line between MgO and Y4Zr3O12 in the temperature range between 1493 and 1573 K was confirmed. The eutectic melting in the MgO-Y2O3-ZrO2 system was established using DTA followed by SEM/EDX microstructure investigation. Based on the obtained experimental results, the thermodynamic database was derived.