Updated project metadata. The clinical performance of trastuzumab in the treatment of ErbB2-positive gastric cancer is severely hampered by the emergence of molecular resistance. The glycosylation landscape of ErbB2’s extracellular domain, and the molecular mechanisms through which it tunes gastric cell malignancy, including the acquisition of trastuzumab resistance, remain elusive. We show that the expression of ErbB2 sialylated glycoforms holds clinical utility in the prediction of clinical outcome and stratification of gastric cancer patients. In-depth glycoproteomic and glycomic analysis of ErbB2 extracellular region disclosed a site-specific profile in gastric cancer cells. We further demonstrate that ST6Gal1 sialyltransferase specifically targets ErbB2 N-glycosylation sites within the trastuzumab binding domain. Moreover, the abrogation of ST6Gal1-mediated α2,6-sialylation reshapes ErbB2 glycome and sensitizes gastric cancer cells to trastuzumab-induced cytotoxicity through receptor membrane stabilization and a downregulation of ErbB2 activation. Overall, this data demonstrates that aberrant sialylation tunes the molecular resistance of ErbB2-driven gastric cancer cells to trastuzumab.