Breast cancer (BC) remains the most prevalent cancer among women and a leading cause of cancer-related mortality worldwide, posing a significant threat to public health. Rare earth (RE)-doped upconversion nanoparticles (UCNPs) have emerged as a promising nanoplatform for BC management, owing to their exceptional photophysical properties and design flexibility. Unlike conventional fluorescent probes, engineered UCNPs absorb near-infrared (NIR) light, enabling deep tissue penetration while mitigating tissue damage and spontaneous fluorescence interference. Furthermore, through core-shell structure engineering and functionalization, multiple diagnostic and therapeutic modules can be integrated within a single NP, enabling theranostic applications for BC. This review comprehensively summarizes recent advances in engineered UCNPs for BC theranostics. It begins by introducing the luminescence mechanisms, controllable synthesis methods, and surface modification strategies of UCNPs. Next, it explores the fundamental biological effects of UCNPs, including biodistribution, metabolic pathways, and biotoxicity. Subsequently, we systematically review applications of engineered UCNPs in BC molecular imaging, biomarker detection, phototherapy, smart drug/gene delivery, and immunotherapy. Finally, current challenges and clinical translation prospects of UCNPs are discussed.