Grain growth has been studied in polycrystalline thin films of Ge, Si, and Au during ion bombardment. The phenomenon has been characterized by varying the ion dose, ion energy, ion flux, ion species, substrate temperature, and thin‐film deposition conditions. Films bombarded with Si+, Ar+, Ge+, Kr+, and Xe+exhibited enhanced grain growth which was weakly temperature dependent and proportional to the energy deposited in elastic collisions at or very near grain boundaries. The effect of these parameters on grain size and microstructure was analyzed both qualitatively and quantitatively using transmission electron microscopy. A transition state model describing the motion of grain boundaries during ion bombardment has been applied to the present experimental data. The results suggest that bombardment‐enhanced grain growth may be due to thermal migration of bombardment‐generated defects across the boundary. The calculated defect yield per incident ion was found to be directly related to enhanced grain growth, and was used to estimate the number of atomic jumps at the grain boundary per defect generated. Grain growth rates during bombardment and thermal annealing were related to their respective point defect populations.