Here we describe the 3D structures of β2m amyloid fibrils obtained by cryo-EM and image processing by sorting fibril segments into homogeneous subsets. The 3D maps and mass analysis by scanning transmission electron microscopy (STEM) reveal a novel dimer-of-dimers repeat unit in a complex, hierarchical fibril assembly. We suggest that a globular repeat unit may be a common feature of amyloid fibrils formed from larger protein precursors. The results reveal a new view for amyloid, with a more elaborate superstructure than any previously described.