This work reports the experimental evidence of localized solid-state amorphization (SSA) in a nanostructured Al–Mg alloy processed by high pressure torsion at room temperature. Electron backscatter diffraction analysis indicated that the deformed alloy had a very small average grain size of about 79 nm. High-resolution transmission electron microscopy (HRTEM) observations illustrated that the deformation-induced SSA were frequently located in the vicinity of grain boundaries (GBs) and GB junctions where high density dislocations, severe lattice distortion, deformation twins and stacking faults coexisted in the deformed alloy. The SSA phenomenon may primarily be attributed to the strong interactions of the high dislocation densities, GBs and the planar interfaces. A possible formation process of amorphization is proposed based on the HRTEM investigations. The present results suggest that the crystalline-to-amorphous transformation could also occur in binary Al–Mg alloys through severe plastic deformation that are usually produced by rapid solidification.