Tailoring of a micrometer-long one-dimensional (1D) array of superparamagnetic iron oxide nanoparticles (SNPs) was achieved by Mg(2+)-mediated supramolecular polymerization of a SNP-containing chaperonin protein (GroELMC⊃SNP). The inclusion complex GroELMC⊃SNP formed when ligand-modified SNPs were mixed with GroELMC, a GroEL mutant having multiple merocyanine (MC) units at its apical domains. Upon mixing with MgCl2 in phosphate buffer, GroELMC⊃SNP polymerized via the formation of multiple MC-Mg(2+)-MC coordination bonds, yielding thermodynamically stable micrometer-long nanotubes encapsulating 1D-arrayed SNPs (NTGroEL⊃SNP). When the NTGroEL⊃SNP nanotubes in phosphate buffer were incubated in a 0.5 T magnetic field, they began to assemble laterally and then organized into thick 1D bundles, where longer nanotubes were more preferentially incorporated. When the applied magnetic field was turned off, such bundles disassembled back to the individual 1D nanotubes. Lateral assembly of 1D SNP arrays in a magnetic field has been theoretically predicted but never been proven experimentally.