Galectin-9 (Gal-9, LGALS9) is a member of the family of carbohydrate-binding lectins known as galectins. Galectins bind a diverse repertoire of galactose-bearing glycoprotein receptors expressed across multiple cell types. These interactions elicit a broad spectrum of pleiotropic effects important in both normal physiology and disease pathogenesis. Gal-9 contains two separate carbohydrate recognition domains with overlapping yet also divergent binding affinities for distinct glycostructures. This tandem repeat motif enables fine-tuning of its various biological functions. Additional control of Gal-9 activity is provided via multiple gene variants, protein isoforms, tissue distribution, and cell type-associated glycoprotein binding profiles. Within the tumor microenvironment, Gal-9 interacts with immune, non-immune, and cancer cells to influence malignant progression. Its binding of the premier immune checkpoint glycoreceptors, T-cell immunoglobulin and mucin-domain-containing-3 (TIM-3) and programmed cell death protein 1 (PD-1), places Gal-9 apart as a burgeoning target for immunotherapy. In this review, we delve into important aspects of Gal-9 immunobiology in tumorigenesis, including glycobiological and lineage-dependent functions. We further examine Gal-9 as a promising new glyco-immune checkpoint target for cancer therapy.