Nanotechnology has significantly advanced cancer therapy, particularly through the development of multifunctional nanoparticles (NPs) capable of acting as both therapeutic and diagnostic agents. This review focuses on the synergistic integration of radiotherapy (RT) and photothermal therapy (PTT) mediated by engineered NPs-a rapidly evolving strategy that enhances tumor specificity, minimizes healthy tissue damage, and enables real-time imaging. By analyzing the recent literature, we highlight the dual role of NPs in amplifying radiation-induced DNA damage and converting near-infrared (NIR) light into localized thermal energy. The review classifies various metal-based and composite nanomaterials (e.g., Au, Pt, Bi, Cu, and Fe) and evaluates their performance in preclinical RT-PTT settings. We also discuss the physicochemical properties, targeting strategies, and theragnostic applications that contribute to treatment efficiency. Unlike conventional combinatorial therapies, NP-mediated RT-PTT enables high spatial-temporal control, immunogenic potential, and integration with multimodal imaging. We conclude with the current challenges, translational barriers, and outlooks for clinical implementation. This work provides a comprehensive, up-to-date synthesis of NP-assisted RT-PTT as a powerful approach within the emerging field of nano-oncology.