It is becoming more and more important to effectively integrate multiple complementary diagnostic imaging and synergistic therapies into a nano-platform, but it is still challenging. Here, we used bovine serum albumin (BSA) as a template to prepare ultra-small Ag/Gd2O3 (Ag/Gd@BSA) hybrid nanoparticles with high water dispersion by a biomineralization strategy for magnetic resonance (MR)/computed tomography (CT)/photoacoustic (PA) imaging-guided photothermal therapy (PTT). Compared with commercial MR and CT contrast agents, we showed that these well-characterized BSA-templated nanotheranostics possessed higher r1 relaxivity (5.84 mM-1 s-1) and HU values. In addition, these nanotheranostics have an excellent NIR absorption feature and outstanding photothermal conversion efficiency (47.4%) in the solution phase. The in vivo imaging experiment demonstrated that the Ag/Gd@BSA hybrid nanoparticles could serve as tri-modality imaging contrast agents to enable precise diagnosis of tumors. Meanwhile, it was also revealed that Ag/Gd@BSA had ability to be an ideal nanotherapeutic agent to achieve a satisfactory tumor treatment effect through PTT. Also, we showed the good biocompatibility of Ag/Gd@BSA nanoparticles. Overall, these results indicated that Ag/Gd@BSA was an effective nanotheranostic, which could accurately identify tumor sites and realize complete tumor elimination, having great potential in clinical transformation.