Metastatic breast cancer (MBC) remains the principal cause of mortality among women with breast malignancies, largely because microscopic deposits evade early detection and resist systemic therapy. Magnetic nanoparticles, especially superparamagnetic iron-oxide nanocrystals functionalized with targeting ligands and therapeutic cargos are emerging as versatile theranostic agents poised to transform this landscape. Owing to their strong magnetization and biocompatibility, these particles markedly amplify T₂-weighted MRI contrast, illuminating occult nodal and visceral metastases that conventional imaging often misses. Conjugation with antibodies, peptides, or small-molecule inhibitors enables active homing to overexpressed receptors such as HER2 and CXCR4, sharpening lesion delineation while simultaneously enriching drug payloads at metastatic foci. Once localized, externally applied alternating magnetic fields induce nanoscale friction that heats the tumor microenvironment, triggering controlled hyperthermia capable of sensitizing cancer cells to chemotherapeutics and radiotherapy or, at higher temperatures, inducing direct cytotoxicity. The same magnetic guidance can steer nanoparticles across physiological barriers, concentrate them in hard-to-reach bone or brain metastases, and minimize off-target exposure, thereby lowering systemic toxicity. Surface engineering further permits co-loading of siRNA, immune agonists, or photosensitizers, supporting multimodal regimens that synchronize imaging, drug delivery, hyperthermia, and real-time response monitoring within a single platform. Early-phase clinical trials already demonstrate improved sentinel-node detection and encouraging safety profiles, yet hurdles remain: large-scale reproducibility, long-term biodistribution, and regulatory harmonization across diagnostic and therapeutic classifications. This chapter provides an in-depth analysis of the current state of the field, covering topics such as magnetic-core fabrication, ligand conjugation strategies, imaging physics, therapeutic mechanisms, immune-oncology synergies, translational case studies, toxicological challenges, and future prospects. It serves as a comprehensive guide for clinicians, materials scientists, and nanomedicine innovators on the deployment of magnetic nanoparticles for the visualization, targeting, and treatment of metastatic breast cancer.