Commonly, the increased free surface of nanometals results in completely different mechanical behaviors from their bulk counterparts. At present, studies on the plasticity behavior of nanometals is widely carried out under quasi-static states. Understanding the plasticity mechanism of nanometals during a high-speed forming process is largely unexplored. This study explored the rate dependence of the forming behaviors of Al and Au nanofilms using laser-induced ultrahigh strain rate forming processes. The results showed that the superplastic behavior of the nanofilms can be activated above a critical value of the strain rate (>2.0E8 s−1). The Al nanofilm exhibited a maximum vertical strain of ∼ 567% at a strain rate of 8.1E8 s−1, and that of the Au nanofilm was ∼ 620% at a strain rate of 8.8E8 s−1. The superplastic forming mechanism mediated by interstitials was revealed for the first time. Further, the potential contribution of the interstitial-mediated plasticity mechanism in breaking through grain size limit and constitutive model modification was discussed. The discovery of this particular mechanism supplements the deformation mechanism diagram and constitutive relationship of nanometals, and is thus of great significance to the study of material responses in extreme conditions and manufacturing process analysis and optimization.