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A biocompatible Bi@mSiO 2 @MnO 2 nanocomposite is fabricated using a facile stepwise reaction method, which exhibits powerful theranostic performance for CT/MR imaging-guided enhanced PTT/CDT/chemotherapy of tumours. [Display omitted] Cancer is a leading cause of death worldwide and seriously threatens the health of humans. The current clinical treatments for cancer are not efficient and always lead to significant side effects. Herein, a biocompatible and powerful theranostic agent (Bi@mSiO 2 @MnO 2 /DOX) is fabricated using a facile stepwise reaction method. The Bi nanoparticles (NPs) are coated by mesoporous silica to protect the Bi NPs from oxidation, which guarantees the stable photothermal effect of the Bi NPs. When the Bi@mSiO 2 @MnO 2 /DOX nanocomposites (NCs) accumulate in the tumour site, hyperthermia is generated by Bi NPs under near-infrared (NIR) light irradiation for photothermal therapy (PTT), and the generated heat triggers the release of DOX for chemotherapy in the tumour. In addition, the MnO 2 of the NCs responsively catalyses endogenous H 2 O 2 to generate O 2 , raising the oxygen level to enhance the effect of chemotherapy in the tumour microenvironment (TME), and consumes glutathione (GSH) to produce Mn2+ for magnetic resonance (MR) imaging. Under acidic TME conditions, H 2 O 2 and Mn2+ also produce toxic hydroxyl radical (· OH) for chemodynamic therapy (CDT). Furthermore, the Bi NPs can also be used as excellent contrast agents for X-ray computed tomography (CT) imaging of tumours with a high CT value (6.865 HU mM−1). The Bi@mSiO 2 @MnO 2 /DOX NCs exhibit a powerful theranostic performance for CT/MR imaging-guided enhanced PTT/CDT/chemotherapy, which opens a new prospect to rationally design theranostic agents for tumour imaging. [ABSTRACT FROM AUTHOR]