Tailoring of a micrometer-long one-dimensional (1D) array of superparamagnetic iron oxide nanoparticles (SNPs) was achieved by Mg2+-mediated supramolecular polymerization of a SNP-containing chaperonin protein (GroEL(MC)superset of SNP). The inclusion complex GroEL(MC)superset of SNP formed when ligand-modified SNPs were mixed with GroEL(MC), a GroEL mutant having multiple merocyanine (MC) units at its apical domains. Upon mixing with MgCl2 in phosphate buffer, GroEL(MC)superset of SNP polymerized via the formation of multiple MC-Mg2+-MC coordination bonds, yielding thermodynamically stable micrometer-long nanotubes encapsulating 1D-arrayed SNPs (NTGroEL superset of SNP). When the NTGroEL superset of 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.