We perform an assessment of the Li2O${\rm Li}_2{\rm O}$‐La2O3${\rm La}_2 {\rm O}_3$═ZrO2${\rm ZrO}_2$ phase diagram using an array of computational methods to address significant gaps in available experimental data at high‐temperatures, due to lithium loss and high reactivity with most materials, which poses significant containment challenges. Free energies of the solids are calculated via the quasi‐harmonic approximation (QHA) and thermodynamic integration, based on the density functional theory (DFT) simulations. Free energies of the liquids are modeled as regular solutions, with enthalpies of mixing calculated from DFT molecular dynamics simulations. The melting temperatures of Li2O${\rm Li}_2{\rm O}$, LiLaO2${\rm LiLaO}_2$, La2Zr2O7${\rm La}_2 {\rm Zr}_2 {\rm O}_7$, and Li7La3Zr2O12${\rm Li}_7 {\rm La}_3 {\rm Zr}_2 {\rm O}_{12}$ (LLZO) are calculated using the small‐size coexistence method. Free energies from prior experimental assessments are combined with our computational results to generate a CALculation of PHAse Diagram (CALPHAD) model and phase diagrams.