Photothermal therapy (PTT) is a novel therapy in the treatment of cancer which bases on the conversion of photon energy into heat (>43^oC), and photoacoustic imaging (PAI) is a novel imaging technique for cancer diagnostic and therapeutic monitoring. To enhance the treatment depth of PTT and the signal of PAI, the near-infrared (NIR)-absorbing photothermal agents have been used. The development of NIR-absorbing photothermal agents with excellent properties such as highly photothermal stability, high absorption in the NIR region, and excellent biocompatibility is needed in modern biomedicine. Gold nanostar (AuNS) has potential applications in the biomedical field owing to the large NIR absorption and the high NIR photothermal effect. However, the reported methods to synthesis AuNS are complicated and toxic, which is a serious issue for its practice. In this work, a novel green synthesis procedure using chitosan polymer from marine and vitamin C for the preparation of AuNS was introduced. Chitosan was used as a stabilizer, shape-directing, and size-controllable agent for the preparation of AuNS for the first time. The obtained AuNS sized from 111 to 225 nm with different absorption peaks. The AuNS exhibited NIR absorption with excellent biocompatibility toward non-cancerous and cancerous cell lines. The in vitro tests proved the high efficiency of obtained AuNS on both PTT and PAI.