Triple-negative breast cancer (TNBC) comprises 10-15% of breast tumors, and lacks targeted therapy. Galectin-8 (Gal-8) is a tandem-repeat type galectin involved in cell adhesion and migration, angiogenesis and tumor progression. Here, we studied the tumorigenic properties of Gal-8 and its ligand ALCAM/CD166 in TNBC. We silenced both Gal-8 and ALCAM in MDA-MB-231 cells with specific (MDA-shGal8 and MDA-shALCAM, respectively) or scrambled shRNA lentiviral particles (MDA-shControl). Interestingly, both MDA-shALCAM (p<0.01) and double silenced MDA-shALCAM-shGal8 (p<0.001) cells showed decreased proliferation and Bcl-2 down-regulation (p<0.001) compared to control cells. Moreover, ALCAM-silenced cells showed impaired ability (p<0.001) to form anchorage-dependent colonies and tumor spheres. Silencing of ALCAM decreased cell adhesion and migration onto Gal-8-coated surfaces in a glycan-dependent fashion. Remarkably, either Gal-8 or ALCAM silencing significantly disrupted (p<0.05) cell-cell adhesion. In vivo, in a TNBC experimental model, at 56 days post-inoculum (pi), MDA-shGal8 (p<0.05) and ALCAM-silenced (p<0.01) cells generated smaller tumors than control cells. Notably, at day 98 pi, tumors generated by MDA-shALCAM-shGal8 cells were even smaller (p<0.05) than those generated by MDA-shALCAM cells. In summary, dual knock-down of Gal-8 and ALCAM induced a pronounced delay on tumor growth. Further studies are needed to elucidate Gal-8 and ALCAM mechanisms inducing TNBC.