Abstract: Photothermal therapy (PTT), which uses a photothermal agent (PTA) to transform light to heat to kill tumor cells under near-infrared (NIR) irradiation, is a new non-invasive treatment of cancer and has advantages such as good targeting and negligible side effects compared to traditional treatment methods including surgery, radiotherapy, and chemotherapy. Driven by the rapid development of nanomedicine, PTA has become the core component of PTT. Its properties such as solubility, biosecurity, and photothermal conversion efficiency have direct influence on PTT. Furthermore, PTT can achieve synergetic treatment by combining chemotherapy, photodynamic therapy, gene therapy, and immunotherapy. This review aims to summarize PTA and discuss the progress of PTT in tumor treatment.