The cesium lead triiodide (CsPbI3) perovskite is a promising candidate for stable light absorbers and red-light-emitting sources due to its outstanding stability. Phase engineering is the most important approach for the commercialization of CsPbI3 because the optically inactive nonperovskite structure is more stable than three-dimensional (3-D) perovskite lattices at ambient temperature. This study presents an in-depth evaluation to find the optimum surface ligand and to reveal the mechanism of phase stabilization by surface ligands. Thermodynamic evaluations combined with density functional theory calculations indicate the criteria for forming stable 3-D CsPbI3 perovskites under surface and volume free energy competition between perovskite and nonperovskite phases. Comparative calculations for ammonium, alcohol, and thiol groups show that ammonium groups enhance the phase stability of 3-D perovskites the most. In addition, ammonium-passivated CsPbI3 is relatively robust against defect formation and H2O adsorption.