Tim3 (T cell immunoglobulin and mucin domain protein 3) is a member of the Tim family, containing an immunoglobulin-like domain and a mucin-like domain. Tim3 is mainly expressed on the surface of Th1, Th17 cells, and certain immunosuppressive cells such as regulatory T cells and myeloid-derived suppressor cells. By binding to its ligand, such as galectin-9, Tim3 regulates T cell activation, proliferation, and cytokine secretion, suppressing excessive immune responses. In the tumor microenvironment, Tim3 expression is increased, associated with immune escape mechanisms. The expression of Tim3 is tissue-specific, and mutations or dysregulation are associated with various diseases, including autoimmune diseases, chronic infections, and cancer.