A theranostic nanoagent exhibits great promise to improve diagnostic accuracy and therapy efficacy. Herein, a kind of theranostic nanoagent based on poly(vinylpyrrolidone) (PVP)-protected ultrasmall CuO nanoparticles (CuO NPs) is developed by a facile liquid reduction method. Attributed to high near-infrared absorbance and good biocompatibility, CuO NPs have shown significant potential for photothermal therapy. Moreover, CuO NPs with a satisfactory relaxivity coefficient () can be well applied as outstanding MRI contrast agents and exhibit excellent magnetic resonance imaging (MRI) ability. treatments further demonstrate that CuO NPs could be well used as multifunctional theranostic nanoagents, which achieve precise MRI and a high photothermal antitumor effect. It is expected to further promote the research and application of copper-based nanoparticles as theranostic nanoagents for cancer therapy.