Ultrathin transition metal dichalcogenides (TMDs) seem to have a promising future in the field of theranostic agents due to their excellent near-infrared light absorption capacity and large specific surface area. Plenty of previous studies focused on the therapeutic effects of the materials, but were less concerned with the detailed studies of biocompatibility for clinical transformation. In this work, ultrathin WS₂ nanosheets coated with bovine serum protein (BSA) (WS₂@BSA NSs) were selected as experimental subjects with favorable biocompatibility to explore their potential as a theranostic agent. Firstly, ultrathin WS₂ nanosheets were prepared by ultrasound-assisted exfoliation using n-methyl pyrrolidone (NMP) as the liquid phase, followed by coating with bovine serum protein. The physical and chemical properties of WS₂@BSA NSs were investigated. Secondly, the biocompatibility experiments that are most relevant to clinical transformation were divided into cell level experiments and in vivo experiments with zebrafish as the model organism. Finally, to explore further applications for the diagnosis and treatment of tumors, the in vitro photothermal effect and the X-ray computed tomography (CT) imaging capability of WS₂@BSA NSs were investigated. The obtained results were promising in terms of biocompatibility and theranostics, which suggested the potential of WS₂@BSA NSs for use as a multifunctional theranostic agent in clinics.