Abstract Diblock copolypeptide amphiphiles have been attracting much interest as motifs for self-assembly in the production of nanostructures such as nanoparticles, nanosheets, and nanofibers. In this study, we first focus on the self-assembly of magnetic metal complexes in water using diblock copolypeptides. In addition, not only were nanocomposites formed, but also observed were the development of lower critical solution temperature (LCST) and LCST-induced spin crossover phenomenon, which are discussed. Three composites composed of the iron complex [FeII(ppi)2(NCS)2] (ppi = N-phenyl-2-pyridinalimine) and the diblock copolypeptide amphiphiles 1 and 2 or the polypeptide 3 (containing glutamic acid and leucine) were prepared. Supramolecular structures such as sheets and rectangular morphologies were obtained from composites. A perfectly reversible spin crossover with LCST was successfully generated in the case of composites made with 1. The technique of combining polypeptide molecules and discrete coordination compounds thus makes it possible to design flexible, reversible, and spin-controllable metastable systems.