Galectin family members have been shown to exert multiple roles in the context of tumor biology. Several recent findings support a similar multi-faceted role for galectin-9. Galectin-9 expression is frequently altered in cancer as compared to normal tissues. In addition, an increasing amount of evidence suggests that galectin-9 is involved in several aspects of tumor progression, including tumor cell adhesion and survival, immune escape and angiogenesis. Also, galectin-9 shows potential as a prognostic marker and a therapeutic target for several malignancies. In this review we summarize both the established and the emerging roles of galectin-9 in tumor biology and discuss the potential application of galectin-9 in anti-cancer therapy.