Tin fluoride (SnF 2 ) is widely used as an effective additive for lead‐free tin‐based perovskite solar cells. However, the function of SnF 2 and the mechanism in improving the film morphology are still not clear. In this work, it is clearly demonstrated that SnF 2 can play a crucial role in the crystal nucleation process. Due to the limited solubility, SnF 2 creates more nucleuses for the crystal growth and therefore enables more uniform thin film with high coverage. It is confirmed that this mechanism can be applied to the growth of both thin film and single crystal. As a result of tin‐fluoride‐assisted heterogeneous nucleation, an MASnIBr 2 ‐based perovskite solar cell with a high and stable power conversion efficiency of 3.70% is demonstrated.