Photoacoustic imaging, a novel biomedical imaging technique that combines the advantages of optical imaging and acoustic imaging, offers high-resolution biological tissue imaging to facilitate the observation of deeper imaging sites. In other words, it breaks the “soft limit” of conventional optical bioimaging techniques. However, many diseases, especially in the early stage, present no obvious photoacoustic contrast; therefore, it is crucial to identify effective exogenous photoacoustic contrast agents. Here we introduce a novel two-dimensional material, antimonene nanoflakes (AMNFs), which demonstrates great optical absorption from 300 nm to 900 nm as well as excellent photothermal conversion efficiency and photoacoustic performance. This material is expected to be useful as a contrast agent, helping to achieve excellent photoacoustic imaging of ultra-small tumors in vivo.