<p>Radiotherapy is one of the most effective approaches to treat malignant tumors. Nanoparticles-based technologies are considered to be advanced strategies in radiotherapy due to their ability to both increase effectiveness and decrease side effects of the therapy. Unique physiochemical properties of cerium fluoride nanoparticles are making them prospective radiosensitizers. In this study, radiosensitizing activity of Bi-Tb-doped cerium fluoride nanoparticles (Ce_0.75Bi_0.15Tb_0.1F₃ NPs) under X-ray irradiation was assessed on a 3D tumor spheroid model, based on EMT6/P cells. We showed a pH-dependent radiocatalytic activity of Ce_0.75Bi_0.15Tb_0.1F₃ NPs under X-ray irradiation, demonstrating a 40-fold increase in their prooxidant activity under acidic conditions. Radiosensitizing effect of Ce_0.75Bi_0.15Tb_0.1F₃ NPs was assessed via evaluation of tumor cell migration, viability, and clonogenic activity under X-ray irradiation. Combined action of Ce_0.75Bi_0.15Tb_0.1F₃ NPs (at a concentration of 1&#xa0;mM) and X-ray irradiation (at a dose of 6&#xa0;Gy) caused additive effect (coefficient of drug interaction (CDI) = 1.02), resulting in a decrease of cell migration from irradiated 3D tumor spheroid. Moreover, the combined treatment decreased both viability and proliferation of tumor cells synergistically (CDI = 0.63). Thus, Ce_0.75Bi_0.15Tb_0.1F₃ NPs demonstrated the synergistic radiosensitizing effect in the 3D tumor spheroid model, suggesting the potential for using these nanoparticles to enhance the effectiveness of tumor radiotherapy.</p> Graphical abstract <p>Bi-Tb-doped cerium fluoride nanoparticles (Ce_0.75Bi_0.15Tb_0.1F₃ NPs) radiosensitizing potential was assessed in tumor spheroid model. Combination of X-ray irradiation and nanoparticle treatment caused significant decrease in survivability and viability of spheroid cells due to synergy of these treatments. Ce_0.75Bi_0.15Tb_0.1F₃ nanoparticles were demonstrated to be prospective radiosensitizers</p> <p></p>