This study provides the first proof of the novel application of bismuth oxide as a radiosensitiser. It was shown that on the highly radioresistant 9L gliosarcoma cell line, bismuth oxide nanoparticles sensitise to both kilovoltage (kVp) or megavoltage (MV) X-rays radiation. 9L cells were exposed to a concentration of 50 μg.mL−1 of nanoparticle before irradiation at 125 kVp and 10 MV. Sensitisation enhancement ratios of 1.48 and 1.25 for 125 kVp and 10 MV were obtained in vitro, respectively. The radiation enhancement of the nanoparticles is postulated to be a combination of the high Z nature of the bismuth (Z = 83), and the surface chemistry. Monte Carlo simulations were performed to elucidate the physical interactions between the incident radiation and the nanoparticle. The results of this work show that Bi2O3 nanoparticles increase the radiosensitivity of 9L gliosarcoma tumour cells for both kVp and MV energies. Monte Carlo simulations demonstrate the advantage of a platelet morphology.