We present the effect of various extents of nanoconfinement on methylammonium lead iodide (MAPbI₃) crystallization, polymorphism, and stability. Specifically, MAPbI₃ precursors were spin coated from co-solutions onto flat SiO₂/Si substrates, vertically oriented AAO nanorods and nanoporous AAO templates. Two-dimensional X-ray diffraction (XRD) patterns of MAPbI₃ films revealed formation of the tetragonal phase on SiO₂/Si substrates and AAO nanorods at room temperature. Surprisingly, the cubic phase of MAPbI₃, typically observed only at temperatures above 60^oC, was present within AAO nanopores. Temperature-dependent XRD analysis and micro photoluminescence revealed that the confinement of MAPbI₃ crystals retarded polymorph phase transitions. Nanoconfinement was further observed to improve the stability of MAPbI₃ crystals exposed to air. MAPbI₃ deposited on SiO₂/Si degraded to PbI₂ completely after only 21 days. Nanoconfined MAPbI₃ crystals formed on nanorod-coated SiO₂ and in nanoporous AAO templates remained stable for more than ten times longer. Our current findings suggest nanoconfinement as a strategy to improve the stability of metal-halide perovskites for solar cell applications.