Tryptophan 2,3-dioxygenase (TDO2) is one of the key enzymes that mediates the catabolism of tryptophan along the kynurenine pathway. Numerous studies have shown that TDO2 is aberrantly overexpressed in a variety of malignant tumor cells, and is closely associated with tumor progression, immune evasion, and immune suppression within the tumor microenvironment. Consequently, TDO2 has become an essential target that has attracted significant attention for tumor immunotherapy. In recent years, TDO2 inhibitors have been proposed to reverse the immunotolerant state of tumors, enhancing immune responses in the host, thus showing potential in cancer therapy. This article provides an overview of the structure and function of TDO2, as well as its role in immunoregulation, with a particular focus on the applications of TDO2 inhibitors in cancer therapy and the latest research advancements.