Many human diseases are associated with amyloid fibril deposition, including type 2 diabetes mellitus where human islet amyloid polypeptide (hIAPP) forms fibrils in the pancreas. We report here that engineered, soluble forms of the human Ca(2+)-binding protein nucleobindin 1 (NUCB1) prevent hIAPP fibril formation and disaggregate preexisting hIAPP fibrils. Scanning transmission electron microscopy (STEM) and atomic force microscopy indicate that NUCB1 binds to and stabilizes heterogeneous prefibrillar hIAPP species. The NUCB1-stabilized prefibrillar species were isolated by size-exclusion chromatography and analyzed by STEM, dynamic light scattering, and multi-angle light scattering. The stabilized prefibrillar species show a size range of 2-6 million Da and have other similarities to hIAPP protofibrils, but they do not progress to become mature fibrils. The effects of NUCB1 are absent in the presence of Ca(2+). We postulate that the engineered forms of NUCB1 prevent hIAPP fibril formation by a mechanism where protofibril-like species are "capped" to prevent further fibril assembly and maturation. This mode of action appears to be different from other protein-based inhibitors, suggesting that NUCB1 may offer a new approach to inhibiting amyloid formation and disaggregating amyloid fibrils.