Significant attenuation and overheating, caused by the absorption of the excitation band (980 nm) in water, are the major obstacles in the in vivo application of lanthanide-doped upconversion nanoparticles (UCNPs). Therefore, appropriately- structured Nd³⁺-doped UCNPs with 808 nm excitation could be a promising alternative. Herein, we developed core–shell–shell structured Nd³⁺-sensitized UCNPs as imaging agents, and decorated them onto the surface of polydopamine (PDA) to construct a novel multifunctional core/satellite nanotheranostic (PDA@UCNPs) for in vivo imaging guidance photothermal therapy using single 808 nm laser irradiation. The core–shell–shell structured design enabled outstanding upconversion luminescence properties and strong X-ray attenuation, thereby making the nanocomposites potential candidates for excellent upconversion luminescence/computed tomography dual modal imaging. In addition, the PDA core not only provides high photothermal conversion efficiency and outstanding antitumor effect, but also endows the platform with robust biocompatibility owing to its natural features. Therefore, this multifunctional nanocomposite could be a promising theranostic in future oncotherapy, with high therapeutic effectiveness but low side effects. This study would stimulate interest in designing bio- application-compatible multifunctional nanocomposites, especially for cancer diagnosis and treatment in vivo.