Amorphous Si is nucleated heterogeneously at grain boundaries during irradiation of polycrystalline Si thin films by 1.5 MeV Xe+ions for temperatures of 150–225 °C. Moreover, the heterogeneous nucleation kinetics of amorphous Si are strongly dependent on grain boundary structure. Following formation at grain boundaries, the amorphous Si layer grows at a rate comparable to that previously observed at a pre‐existing planar amorphous‐crystal interface. As amorphization proceeds, a decrease in average grain size and a marked change in the grain size distribution results. We suggest a simple atomistic model for amorphous phase formation in Si in which the nucleation kinetics are dependent on the point defect‐grain boundary interactions.