Abstract Purpose: Galectin-9, a member of the β-galactoside–binding galectin family, induces aggregation of certain cell types. We assessed the contribution of galectin-9 to the aggregation of breast cancer cells as well as the relation between galectin-9 expression in tumor tissue and distant metastasis in patients with breast cancer. Experimental Design: Subclones of MCF-7 breast cancer cells with high or low levels of galectin-9 expression were established and either cultured on plastic dishes or transplanted into nude mice. The tumors of 84 patients with breast cancer were tested for galectin-9 expression by immunohistochemistry. The patients were followed up for 14 years. Results: MCF-7 subclones with a high level of galectin-9 expression formed tight clusters during proliferation in vitro, whereas a subclone (K10) with the lowest level of galectin-9 expression did not. However, K10 cells stably transfected with a galectin-9 expression vector aggregated in culture and in nude mice. Ectopic expression of galectin-9 also reduced MCF-7 cell adhesion to extracellular matrix proteins. Tumors of 42 of the 84 patients were galectin-9 positive, and those of 19 of the 21 patients with distant metastasis were galectin-9 negative. None of the 13 patients with galectin-9–positive tumors and lymph node metastasis up to level II manifested distant metastasis. The cumulative disease-free survival ratio for galectin-9–positive patients was more favorable than that for the galectin-9–negative group (P < 0.0001). Multivariate analysis revealed that galectin-9 status influenced distant metastasis independently of and to a greater extent than lymph node metastasis. Conclusions: Galectin-9 is a possible prognostic factor with antimetastatic potential in breast cancer.