Relative to bulk material, bismuth oxide nanoparticles (Bi₂O₃-NPs), as spheres or rods, have shown quantum confinement effects and unique optical, electrical, and magnetic properties, as well as high surface reactivity, high refractive index, good photoconductivity, and wide energy band gaps. In this work, we present the first observation and report on the synthesis of Bi₂O₃ nanorings by using a simple and inexpensive sonochemical route. In the presence of trisodium citrate (TSC), the hydrothermal treatment of Bi(NO₃)₃ gives rise to a controlled spherical morphology of Bi₂O₃-NPs, while the absence of TSC leads to the formation of nanorods, highlighting the critical role of the coating layer in influencing the crystal growth of NPs. Subsequently, exposing the spherical Bi₂O₃-NPs to ultrasonication in water led to a rearrangement and self-assembly of toroidally shaped particles, giving rise to the formation of nanorings, as observed through scanning electron microscopy (SEM). The UV-vis analysis demonstrated enhanced absorption of nanorings in the near-infrared (NIR) range compared to other solid structures. Moreover, we demonstrated the direct role of the pore size on the light-absorbing properties of Bi₂O₃ nanorings. Additionally, we showed that the band gap energies of Bi₂O₃ nanoparticles can be effectively tuned, depending on their structure. With the distinctive features and hollow morphology of Bi₂O₃ nanorings, they hold great potential for innovative anticancer treatments. The pronounced absorption of nanorings in the visible and NIR regions can translate into high photothermal conversion efficiency, which can lead to effective hyperthermia-based destruction of the cancer cells. Moreover, the unique hollow architecture of the nanorings makes them highly suitable as excellent nanoscale carriers for chemotherapeutic agents, offering high encapsulation efficiency, as well as stimuli-responsive drug release and targeted delivery against cancer cells.