Theranostic nanomaterials (NMs) have gained increasing attention for their simultaneous performance of diagnosis and therapy. Bi-based NMs hold great potential as theranostic platforms based on their X-ray sensitive capability, near-infrared driven semiconductor properties, and distinctive structures, which facilitate the computed tomography (CT) imaging, photoacoustic (PA) imaging, radiation therapy, and phototherapy. The sophisticated design in composition, structure, and surface fabrication of Bi-based NMs can endow these NMs with more modalities in cancer diagnosis and therapy. In this Minireview, we focus on the recent advances in Bi-based theranostic NMs. A series of unique structures and functions as well as the underlying property-activity relationship of Bi-based NMs are showcased to highlight their promising imaging and therapeutic performance. At the end, we propose some challenges for the design and preparation of Bi-based NMs to improve their cancer diagnostic and therapeutic performance.