<p>Breast cancer remains one of the most common and lethal malignancies among women worldwide. Although conventional treatment approaches—including surgery, radiotherapy, chemotherapy, and targeted therapy—have achieved substantial progress, clinical outcomes are still severely limited by issues such as drug resistance, recurrence, and metastasis. In this context, metal-based immunotherapy has emerged as a novel and highly promising strategy, gaining increasing attention for its unique advantages in enhancing anti-tumor immune responses and remodeling the tumor immune microenvironment. In recent years, mounting evidence has demonstrated that metal nanoparticles, metal–organic frameworks (MOFs), and metal complexes hold great potential in breast cancer immunotherapy. These agents exert immunotherapeutic effects through mechanisms such as immune activation, modulation of immunosuppressive cells, and synergistic enhancement of immune checkpoint blockade. Despite these encouraging developments, several critical challenges remain, including systemic toxicity, limited clinical translation, and insufficient understanding of their immunomodulatory mechanisms. This review provides a comprehensive summary of recent advances in metal-based immunotherapy for breast cancer, with a particular focus on the applications of metal nanoparticles, metal complexes, and metal-based nanocarriers. The mechanisms of action, therapeutic advantages, and existing limitations are thoroughly discussed, and future directions are proposed to facilitate further research and clinical translation in this emerging field.</p>