Cancer is a major disease threatening human health in the twenty-first century, with a rising global incidence. Conventional therapeutic approaches, such as surgery, radiotherapy, and chemotherapy, often fail to achieve curative effects in patients with advanced-stage cancer. Therefore, the development of more precise and effective treatment strategies is essential. X-ray-activated nanoscintillators, an emerging modality, demonstrate significant potential in cancer treatment due to their unique energy conversion capabilities and ability to penetrate deep tissues. This review outlines the fundamental concepts of nanoscintillators and explains the mechanisms underlying their X-ray-excited luminescence. It further explores their applications in oncology, including direct tumor ablation, X-ray energy transduction for photodynamic therapy (X-PDT), and synergistic combinations with chemotherapy. Furthermore, the advantages (such as high penetration depth and multifunctionality) and disadvantages (such as low energy conversion efficiency and biocompatibility concerns) of X-ray-activated nanoscintillators are critically examined. By optimizing matrix materials, dopants, and energy transfer designs, the therapeutic performance of nanoscintillators can be significantly improved. With ongoing advancements, X-ray-activated nanoscintillators hold promising prospects for broad clinical applications, offering patients additional effective treatment options.