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Methylammonium lead iodide (MAPbI 3 ) is an important light-harvesting semiconducting material for solar-cell devices. We investigate the effect of long thermal annealing in an inert atmosphere of compacted MAPbI 3 perovskite powders. The microstructure morphology of the MAPbI 3 annealed samples reveals a well-defined grain boundary morphology. The voids and neck-connecting grains are observed throughout the samples, indicating a well-sintered process due to mass diffusion transfer through the grain boundary. The long 40 h thermal annealing at T = 522 K ( k B T = 45 meV) causes a significant shift in the structural phase transition, stabilizing the low-electrical conductivity and high-efficiency cubic structure at room temperature. The complete disordered orientation of MA cations maximizes the entropy of the system, which, in turn, increases the Pb–I–Pb angle close to 180°. The MA rotation barrier and entropy analysis determined through DFT calculations suggest that the configurational entropy is a function of the annealing time. The disordered organic molecules are quenched and become kinetically trapped in the cubic phase down to room temperature. We propose a new phase diagram for this important system combining different structural phases as a function of temperature with annealing time for MAPbI 3 . The absence of the coexistence of different structural phases, leading to thermal hysteresis, can significantly improve the electrical properties of the solar cell devices. Through an entropy-driven stabilization phenomenon, we offer an alternative path for improving the maintenance, toughness, and efficiency of the optoelectronic devices by removing the microstructural stress brought by the structural phase transformation within the solar cell working temperature range.