Radiotheranostics merges diagnostic imaging and targeted radionuclide therapy to allow treatment of breast cancer to be tailored individually. It employs radiopharmaceuticals and molecular imaging modalities such as PET and SPECT for accurate characterization and treatment of the tumor. The review presents the mechanisms, clinical uses, and future potential of radiotheranostics in breast cancer. The review focuses on the potential of nuclear medicine in driving personalized treatment through biomarker-guided strategies. We discussed recent literature on well-established biomarkers like HER2, ER, and PR as well as novel targets like TROP-2. Theranostic agents like ^177Lu-DOTA-trastuzumab and novel innovations with alpha-emitters as well as dual-targeting nanoparticles are noted. Radiotheranostic agents have enhanced results, most importantly, in HER2-positive patients with metastatic or advanced breast cancer. Therapeutic promise also is brought by recent advances to hard-to-treat subtypes, such as triple-negative breast cancer. While radiotheranostics holds great promise, its wider use in the clinic is hampered by factors like heterogeneity of biomarkers, dosimetry accuracy, and regulatory limitations. Research continues to affirm its revolutionary role in the administration of tailor-made treatment regimens to breast cancer.