The solution processability of photoactive halide perovskites differentiates them from traditional inorganic semiconducting materials that require multiple post-processing steps such as thermal/vacuum/blowing- and solvent-assisted treatment. Here we report a technical breakthrough of isothermally crystallizing high-quality perovskite films at room-temperature (RT) without the necessity of any post-processing. This process takes advantage of our discovery of a metastable intermediate of lower-dimensionality formed by amine-assisted crystallographic lattice expansion from an initial three-dimensional perovskite. Using in situ optoelectrical/chemical and ex situ structural characterizations, a detailed understanding of the low-dimensional metastable intermediate is developed. In conjunction with the metastable intermediate, the rapid evaporation of the solvent and amine facilitates ultra-fast crystallization at RT within seconds. This RT rapidly synthesized perovskite film exhibits a carrier diffusion length of 2.9 μm and {00} preferred orientation with an ultrahigh Lotgering factor of 97%. These films are highly compatible to conventional or inverted devices, demonstrating 22.3% and 23.1% power conversion efficiencies, respectively.