Perovskite solar cells have recently reached staggering efficiencies, through efforts focused on reducing grain boundaries, by enlarging the size of the crystalline domains that constitute the perovskite films. Here, we demonstrate that smaller crystallites within perovskite films spontaneously coalesce into larger ones, even when complete devices are stored in the dark at room temperature. We show that crystal coalescence greatly improves the performance of state-of-the-art perovskite solar cells. Our results reveal the dynamic nature of the morphology of perovskite films and highlight the crucial role that coalescence plays in producing highly efficient devices.