Galectin-9 is a member of the β-galactoside-binding lectin family, characterized by highly conserved carbohydrate recognition domains （CRDs） that specifically recognize and bind to β-galactoside glycans. It consists of two CRDs connected by a linker peptide, forming a unique structure. Galectin-9 plays a significant role in immune regulation by binding to cell surface glycoproteins （such as Tim-3）, thereby modulating the functions of T cells, dendritic cells, and macrophages. It is involved in processes such as immune tolerance, inflammatory responses, and apoptosis. In autoimmune diseases, infections, and cancer, Galectin-9 exhibits dual roles, both suppressing excessive immune responses and promoting anti-tumor immunity. Additionally, Galectin-9 participates in cell adhesion, migration, and signal transduction, and exerts antiviral effects in viral infections （such as HIV and HCV）. Its expression levels are closely associated with the progression and prognosis of various diseases, making it a potential biomarker and therapeutic target.