In the ongoing intense quest to increase the performance of perovskite solar cells, optimizing the morphology of perovskite material has become imperative to achieve high power conversion efficiencies. Despite the fact that nucleation plays a key role in controlling the crystal morphology, very little is known about the nucleation and crystal growth processes. Here, we perform metadynamics simulations of nucleation of methylammonium lead triiodide (MAPI) in order to unravel the atomistic details of perovskite crystallization from a γ-butyrolactone solution. The metadynamics trajectories show that the nucleation process takes place in several stages. Initially, dense amorphous clusters mainly consisting of lead and iodide appear from the homogeneous solution. These clusters evolve into lead iodide (PbI2)-like structures. Subsequently, methylammonium (MA+) ions diffuse into these PbI2-like aggregates triggering the transformation into a perovskite crystal through a solid–solid transformation. These enticing results allowed us to design new experimental strategies to rationally control the dimensions of MAPI grains using the spin-coating technique and to engineer homogeneous nucleation and growth of MAPI single crystals. The experimental results amply support our unprecedented observations related to the critical role of monovalent cations in inducing the nucleation process in lead halide perovskites.