Galectin-1 kills immature thymocytes and activated peripheral T cells by binding to glycans on T cell glycoproteins including CD7, CD45, and CD43. Although roles for CD7 and CD45 in regulating galectin-1-induced death have been described, the requirement for CD43 remains unknown. We describe a novel role for CD43 in galectin-1-induced death, and the effects of O-glycan modification on galectin-1 binding to CD43. Loss of CD43 expression reduced galectin-1 death of murine thymocytes and human T lymphoblastoid cells, indicating that CD43 is required for maximal T cell susceptibility to galectin-1. CD43, which is heavily O-glycosylated, contributes a significant fraction of galectin-1 binding sites on T cells, as T cells lacking CD43 bound approximately 50% less galectin-1 than T cells expressing CD43. Although core 2 modification of O-glycans on other glycoprotein receptors is critical for galectin-1-induced cross-linking and T cell death, galectin-1 bound to CD43 fusion proteins modified with either unbranched core 1 or branched core 2 O-glycans and expression of core 2 O-glycans did not enhance galectin-1 binding to CD43 on T cells. Moreover, galectin-1 binding clustered CD43 modified with either core 1 or core 2 O-glycans on the T cell surface. Thus, CD43 bearing either core 1 or core 2 O-glycans can positively regulate T cell susceptibility to galectin-1, identifying a novel function for CD43 in controlling cell death. In addition, these studies demonstrate that different T cell glycoproteins on the same cell have distinct requirements for glycan modifications that allow recognition and cross-linking by galectin-1.