Contrast agents play a vital role in the enhanced examination of computed tomography (CT) imaging. However, traditional clinical small-molecule agents face a variety of drawbacks, such as low blood circulating time, difficult modification and potential toxic and side effects. Herein, a simple albumin-directed fabrication of platinum (Pt) nanocrystals was achieved for exploring the utilization in CT imaging. Ultrasmall nanoagents with a mean core size of 2.1 nm were obtained through a facile one-pot synthesis by the reduction of chloroplatinic acid hexahydrate (H2PtCl6·6H2O) using bovine serum albumin (BSA) as the biotemplate under room temperature. These synthesized well-dispersed nanocrystals exhibited good haemocompatibility and biocompatibility. Interestingly, it was demonstrated that the nanocrystals could serve as potential new and potent CT contrast agents, especially vital for in vivo imaging with prominent enhancement and metabolizable behaviours due to the combination of the higher X-ray attenuation property and prolonged imaging time, perhaps caused by the BSA modification. Furthermore, such ultrasmall platinum nanocrystals obtained from a feasible mild aqueous synthetic route for CT imaging has not been reported before. Thereby, this work also gives new insights for the protein-templated growth of biocompatible nanoparticulate contrast agents in future nanomedicines.